I'm torn between getting at 50W or 100W thermometer for my 20 gallon tank. I know they both could work for my tank as they are both adjustable, I'm just not sure if one would be better than the other.

Here is the table to help with what size individual heater you need. Delta T is the difference from the lowest ambient room temperature in the room to your desired tank temperature. Always get a heater larger than you think you need.

Here is the table to help with what size individual heater you need. Delta T is the difference from the lowest ambient room temperature in the room to your desired tank temperature. Always get a heater larger than you think you need.

If you plan on buying 100 watts you might consider getting two 50w heaters instead of one 100w.

The primary benefit is that if one overheats (gets stuck in the ON position) it's less likely to boil the fish than if a more powerful heater malfunctions.

A secondary benefit is that you can place them at opposite ends of the tank for more even heat distribution if desired.

Always go a little bigger with your heater if your only running one incase of failure and if you ever have to treat for ick you will want to get your water temp up in the 80's

Two 200W heaters provide a total of 400W of heat. It's a basic principle of physics.

Why the OP in that thread was having issues I don't know. I'd suspect either defective heaters or inadequate flow around the heaters that resulted in them heating up internally and triggering the thermostat to shut off without much of the heat being transferred to the water.

Edit: To expand on my initial statement here, imagine you have a room with two wood stoves. Claiming that two heaters won't combine to make the tank warmer is akin to saying lighting a fire in the second stove won't make the room any warmer than having a fire in just the first stove. The heat from both fires are contributing to making the room warmer. Same with the heaters. As long as both heaters are functioning properly, they should cycle on and off until the water hits the desired temp.

In the linked thread nobody asked the OP enough questions about his setup to determine what the problem might be. Nobody asked how long he was waiting for the temperature to equalize. Depending on water circulation it may not be unusual for the heater to cycle many times before the water hits the desired temp. It wasn't even clear if he'd turned up the settings on the heaters. He may have assumed that setting the heater to 27C was absolutely going to get him 27C in the far corner of the tank. There was certainly not enough actual evidence gathered to logically conclude his problem was the result of having two smaller heaters instead of one large heater.

Sure. This has been discussed in great detail and this is why two heaters of the same value do not work, and cant. It's simple thermodynamics. Here we go again with the proof.

Let’s see how multiple heaters cannot heat a tank to a desired temperature and later how they can heat your tank, but first the math and the science for all.

Remember all heaters work independently of each other; they are not connected in any way. The tank water has no idea how many heaters are running let alone their wattage. All the water temperature knows is how much heat is being lost due to ambient temperature. Remember this.

Let’s assume a perfect world, perfect equipment, and perfect water temperature. This means heaters rated for their temperature to turn off exactly at that set point 100% accurate like they are supposed to, thermometers are 100% accurate, ambient temperature is always the exact same temperature and does not deviate and all within a 0.0 degrees. Does not matter if your heater can only heat up to its maximum temperature or that heater is on a perfect heater controller to turn off at a set temperature, the same occurs.

With the above thermodynamics table we have the following: 150 gallon tank and three heaters available. The ambient temperature in the room is a constant perfect 55 degrees 24/7 all the time all year round. The desired temperature you want the water in the tank needs to be 80 degrees. The heaters can only heat to their delta T (difference) as specified by the above table. As you can see we need one heater to be 500 watts to get to our desired temperature of 80 degrees, 55 degrees ambient + 25 degrees delta T = 80 degrees target temperature.

Heater A = 200 watt, can only raise the temperature to 65 degrees (delta T = 10)

Heater B = 300 watt, can only raise the temperature to 70 degrees (delta T = 15)

Heater C = 400 watt, can only raise the temperature to 75 degrees (delta T = 20)

Heater A+B+C = 900 watts when all heaters are on at the same time.

(Everybody can see the issue already but let’s continue)

Put all heaters in your tank and turn them on. All three heaters are providing 900 watts of energy to heat the tank up very quickly. Once the temperature hits 65 degrees heater A turns off as it cannot heat anymore.

The math: Heater A+B+C = 0 + 300 + 400 = 700 watts heating the water.

The water heats up a little slower now until the temperature hits 70 degrees and now heater B turns off as it cannot heat anymore.

The math: Heater A+B+C = 0 + 0 + 400 = 400 watts heating the water.

The water heats up slower now until the temperature hits 75 degrees and now heater C turns off as it cannot heat anymore.

The math: Heater A+B+C = 0 + 0 + 0 = 0 watts heating. The temperature of the water stays at 75 degrees.

Even if you add a second heater of 400 watts you can see that they both will turn off at 75 degrees.

The desired temperature of 80 degrees cannot be reached until 500 watt heater is used.

Of course all this is in a perfect world works as mentioned.

Let’s see what happens in the real world now.

Heaters don’t heat to their desired maximum temperature, could be more, could be less. The same heater can heat to like 75 degrees and the identical heater added can heat to above 78 degrees thus two heaters can raise the temperature to your 78 degrees. This is how folks say that adding an additional identical heater got their tank to their temperature. Or one heater heats to 78 degrees and the identical heater heats to 76 degrees thus the temperature does not increase. The most likely scenario here is the ambient air temperature has increased.

Thermometers can by like +/- 5 degrees accurate.

Ambient room temperature varies a lot, close to 15 degrees or more from winter to summer so this has to be taken into account, however your mileage will vary here. As you can see if your ambient temperature goes up a few degrees so does your water temperature.

Lights are on. Lights add quite a bit of temperature to a tank via radiant heat.

My tank: 75 gallon, full hood and canopy with only the back open to the air, winter temperature in the room gets to 62 degrees but can get down to 60 degrees. Target temperature 76 degrees. Delta T = 16 degrees.

Heater needed is a little bit larger than 220 watts but if I decide I want 78 degrees, delta T is now closer to 20 so I need a 293 watt heater. I put in a 300 watt heater. Heat in this half of the house goes to 62 degrees at 10pm at up to 68 degrees at 6:30 am each morning. Works just fine.

Summer time the room gets to 74-76 degrees with the air conditioner on. I turn the heater off during the summer but my water rises to 80-82 degrees! How can that be? Simple…

Light are on. Yeah, LED lights on about 12 hours (actually 24/7 LED lights) a day under a canopy that traps the heat so the tank water rises. The only thing I can do is maybe adding some fans to vent the excess heat from under the canopy or open the top of the canopy. However I have found out the 80-82 degrees does not affect the fish at all, so it just stays at 82 degrees 6+ months out of the year.

Oh, one last thing. As you can see the so called 3-5 watts per gallon rule does not work. Using the table above a 10 gallon tank with a delta T = 10 would need about a 50 watt heater, but at a delta T = 25 needs about a 140 watt heater. So (5w x 10g) = 50 watts does not work. So what this means is that if your room temperature is always at about 68 degrees or higher, the 3-5 watts per gallon would work.

This is why when you go to your local fish store their ambient temperature is warm inside like 75 degrees or more. So they don’t have to use that much energy to raise the temperature.

Remember smaller tanks loose heat faster than larger ones so always buy a larger heater than you think you need.

So now you know how heaters actually work with your fish tank.

Here is a perfect example of the above.

Example of 2 heaters not working. Unable to raise the temperature a simple 2-3 degrees C. He needs 350 watts but has two (2) 200 watt identical heaters.

This is a lot of information, thank you! I'm only asking about which heater to get because for awhile I've only had the little dinky heater that came with the tank, and one of those sticker thermometers. I don't feel the sticker is giving me an accurate read, and I don't think the little heater that came with is keeping it warm, and there's no way to adjust it, just one of those plop in and plug in. So I was shopping around for ones I could adjust, and I just wasn't sure how many watts to go, and I thought I had heard something like "get more than you think you need" somewhere before. I know obviously I'm not going to go overboard with that, but I have chosen to go with the 150W, and I may keep the little heater in, depending on how well I think the new one will work. I thank you all again for your responses!

This misunderstanding of delta T is the source of the confusion here. Delta T is a measurement of the ability of a heater to overcome heat loss to the environment. It is not the point at which the heater turns off because it's done all it can do.

The delta T you see for any particular heater is an approximation as the precise calculation is dependent on the specific usage situation. If four sides of your tank are insulated to R-30, you're going to get much less heat loss than an non-insulated tank and any heater you put in that tank will be able to maintain a delta T significantly higher than the conventional rating. You can do situational delta T requirement calculations for an aquarium the same way you can for a house. You just need to know the R value of the glass/acrylic.

Just as you say the tank water has no idea how much total wattage the heaters provide, the heaters have no idea how cold it is in the room. They only know whether the tank is below or at/above the set point. If it's below, they continue to run. If it's at or above, they turn off.

In the scenario you laid out, the 200W heater is not going to shut off when the tank hits 65F. Its delta T of 10F does not mean it will give up at 10F above ambient. It means that even when running non-stop, it can only maintain a maximum delta T of 10F from ambient. When the tank hits 65F it will continue to run and continue to contribute 200W of heat to the tank. Same with the 300W. All three heaters will continue to contribute heat to the tank until they hit their set points.

Now, there is a slight flaw in the way most heaters do this as the temperature sensor that controls the on/off cycle is inside the heater tube. It's going to react to the temperature of the air inside the tube, not to the temperature of the water. When the tank is in a more or less steady state, with the water very close to the set temperature, this doesn't matter. When the water is a lot colder than the set point, the air inside the tube will heat faster than the body of water outside. The heater will come on, run for a while, and then turn off because the air inside the heater tube has reached the set point. Rather quickly the air in the tube will cool as that heat is absorbed into the tank, and then the heater will come on again. It will continue this cycle until the tank is at the set point.

This is why when you first put in a new heater you need to make small adjustments to the set point and then wait hours, or possibly days, for the tank to get very close to the heater's setting. I suspect this was actually the problem for the OP in the other thread. He was expecting the heaters to stay on until the temp hit the set point. The cycling on/off confused him. In a large tank it can take a very long time for the temperature to stabilize at the set point and the heater will cycle on and off until the set point is hit.

Sorry you are incorrect. Can't change the laws of thermodynamics.

When this question was posed this question to Eheim, just to check the math, I received this from Eheim.
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The heaters ability to heat the water will also be based on ambient room temperature. In general a heater will heat the water up 10'F above the ambient room temperature. The set temperature on any given heater will heat within +/- 1'F based on assumed near perfect accuracy, adding additional heaters will help stabilize the temperature of the water but it will not increase the overall ability to heat a given volume of water.
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On this point we are in full agreement. Unfortunately, there seems to be a misunderstanding on how exactly they apply to this situation.

A fundamental principle of physics and thermodynamics is conservation of energy. If you put X amount of energy into a system, regardless of whether it's from one source or a million sources, the total amount of that energy goes into the system. In this particular case the energy is in the form of heat. 500W of heat energy is 500W of heat energy, regardless of whether it's from one 500W heater or ten 50W heaters or five hundred 1W heaters.

I could give you dozens of counter examples that show the line of reasoning you're pursuing here is incorrect. For your basic point to be true, it would have to be true that turning on fifty 100W light bulbs does not produce any more light (or heat) than one 100W light bulb. It would have to be true that one candle produces the same heat as a thousand candles. It would have to be true that one burning charcoal briquette produces the same total amount of heat as five hundred burning charcoal briquettes.

The amount of heat produced by each heater goes into the tank. It has nowhere else to go. As long as that heater is running it will contribute heat to the tank. If it's functioning properly, it will continue to cycle on/off as I described previously, continuing to provide heat to the tank until the target temperature is reached.

As for the response from Eheim, I can only assume whoever sent that to you is misinformed. Even Eheim can't change the laws of physics.

Let's take the easy course. You have 2 identical tanks that are at the same temperature of 70 degrees. Take one tank and pour all the water into the other. What is the temperature of the water now? 70 degrees. It does not go to 140. Pour in as much water as you want at 70 degrees. It stays the same.

Let's take the easy course. You have 2 identical tanks that are at the same temperature of 70 degrees. Take one tank and pour all the water into the other. What is the temperature of the water now? 70 degrees. It does not go to 140. Pour in as much water as you want at 70 degrees. It stays the same.

You also have twice as much water. You are adding heat to the system, but you're spreading that heat over more water so the overall temperature -- the heat energy per unit volume -- remains the same. ((1.0 * 70) + (1.0 * 70))/2.0 = 70. If you heat the second tank to 140 and add it to the first tank, you'll have twice as much water in the first tank at 105, the average of the two temps. ((1.0 * 70) + (1.0 * 140))/2.0 = 105. If the second tank contains half as much 140 degree water as the first tank has 70 degree water, then combining them will give 1.5 times the original volume of the first tank at 93.3 degrees. ((1.0 * 70) + (0.5 * 140))/1.5 = 93.33... Each volume of water brings with it an amount of heat energy that's spread throughout the combined volume.

The aquarium heater is moving energy from your incoming power lines into the heating coils where that energy is converted to heat. It's injecting energy/heat into the system from outside without altering the volume of water, so the heat energy per unit volume goes up. It doesn't matter if it's one coil producing 500W of heat energy or five coils each producing 100W of heat energy or one at 100W, one at 150W, and one at 250W; the heat energy is all going into the water and raising the temperature. This is a fundamental principle of thermodynamics.

There are some oddities with water around the freezing and boiling points due to phase change, but those aren't relevant to this discussion. In the range of temperatures in an aquarium, water temperature responds in a linear fashion to the energy injected into the system.

And you keep forgetting that the heaters are turning off at their preset or maximum temperature or simply staying on because of the heat loss is greater than the heat gained. The simple water tanks above just proves the point. It's like you are saying you have 2 cars that can go 100 mph each but both of them can go 200 mph.

Edit - Oh and the math proves it. You have 2 heaters at 100 watts each. If both are on you have (100+100) watts/2heaters = 100 watts. If just one heater is on and the other is off, (0+100)/1 = 100 watts. If you have 1 heater at 200 watts that is on, (200)/1=200 watts.

Please stop adding more heat to the equation than needed, leaving lights bulbs on all the time with no set temperature to turn off or a wood stove that can't have a set temperature either.

And you keep forgetting that the heaters are turning off at their preset or maximum temperature or simply staying on because of the heat loss is greater than the heat gained.

Think for a moment about what you've said here. "...the heaters are turning off at their preset..." The implication here is that the heater will continue to provide heat until it turns off at the preset. Now ask yourself under what conditions the heater will turn off. When it hits the preset temperature, which is exactly the condition you're hoping to achieve. It will run, admittedly cycling on/off due to what I've described before, but more on than off, until the water temperature hits the preset point. ALL the heaters will do this in exactly the same way. If they're all set to 85F, they'll all continue to run until the water reaches 85F. The 100W won't get tired and quit at 75F, it will simply see that the water isn't yet 85F and will continue to run. The same applies to all the others. The collective heat provided by all the heaters will be added to the tank.

Look, if this wasn't true, how could you possibly heat a 10,000 gallon aquarium? Do you imagine there's somebody out there making 50,000W heaters? The biggest single heater I've seen for the home market is 500W. If the heaters weren't additive you couldn't heat a 300 gallon aquarium to anything more than a couple degrees above ambient. The only way you'd ever be able to have a 500 gallon discus tank would be to keep the room at 85F and hope that evaporation doesn't cool the tank down too much.

Aquarium heaters are additive. It's fundamental physics. I don't know what more I can say about this.

If they're all set to 85F, they'll all continue to run until the water reaches 85F. The 100W won't get tired and quit at 75F, it will simply see that the water isn't yet 85F and will continue to run.

Go back and look at the heating chart. Those are the numbers of what one individual heater can heat to the maximum for a given volume of water and the ambient outside temperature. No where in that chart does it say 2 or more heaters. It cannot heat any more than that number, period. So adding a second identical heater cannot heat any more that that same number. Period. Proven for years.

Tell you what, run a bath and have the water temperature heated to 50 degrees. Stop adding water. Now add any amount of water with the same temperature of 50 degrees. Now get in. Is the water 100 degrees??? Nope.

Try this, boil 2 pots of water to 150 degrees. Now add the two together. The temperature does not go to 300 and turn into steam. Stays at 150.

If this was true the energy crisis would be solved hundreds of years ago.

As for your 10,000 gallon tank... that why they just increase the ambient temperature in the room to what the need. Probably don't have any heaters for the tank itself.

I've hit the limits of my ability to explain thermodynamics. You obviously aren't understanding what I'm saying so there seems little reason to continue the discussion.

A few weeks ago I set up a small 28L fry tank and began cycling it. I added a 50w heater set to 27C. However I neglected to consider ambient room temperature. Unlike my other tanks, this tank is in a room in the house where the temp can drop to 10C on cold nights. After a week it was obvious that the heater could not cope. It was continually cycling off and on and struggling to raise the temp above 25C. I added a second 50w heater also set to 27C. Within a few hours the temp reached 27C and has remained stable since with both heaters working in unison (despite some even colder nights). Total wattage is what matters.

A few weeks ago I set up a small 28L fry tank and began cycling it. I added a 50w heater set to 27C. However I neglected to consider ambient room temperature. Unlike my other tanks, this tank is in a room in the house where the temp can drop to 10C on cold nights. After a week it was obvious that the heater could not cope. It was continually cycling off and on and struggling to raise the temp above 25C. I added a second 50w heater also set to 27C. Within a few hours the temp reached 27C and has remained stable since with both heaters working in unison (despite some even colder nights). Total wattage is what matters.

Well yes that is fully explained in message #10 under 'Let’s see what happens in the real world now' especially since it's such a small tank. Thanks for verification. We are done.

Is there anyone who is not 'done'? I'm genuinely curious about the topic. It's simple to find multiple reputable references to the effect that a single heater can be replaced by two heaters of the same combined wattage. If anyone has alternative references that support the prior speculation in this thread I'd appreciate them, thanks.

If we are "not" done, can I be removed from this thread? As the OP od this thread, my problem was solved after the first response, I don't need to be notified every hour how your arguement is going. Please administrators, mark this thread as resolved.