Heater Dilemma


I have a 5 gallon and 10 gallon tank. I had an older 100w heater that I put in the 10 gallon and used the 50w heater that came with the 10 gallon in the 5g.

The 100w seems like it's being fritzy so I would like to buy a new heater. Question -- is a 100w heater too much for a 10 gallon tank? I'm just not sure if I should buy a 25w for the 5 g and move the 50w back into the 10g, or just buy either another 50w or a 100w and be done with it.

Where I am, a 50 watt heater is only about $1-2 less than 100watt. 25 watt heaters are MORE expensive than either.

Bottom line -- I'm terrified of cooking my fish. Is there less chance of doing that with a lower watt heater, or does a stuck heater present the same danger no matter the wattage?

Brands I'm considering after research are Eheim Jager (can I put this horizontally - that's the only way I think it will fit!) or Aqueon Pro.

Thanks for any input!


Ive have 2 heaters(seprate tanks) that are out by 4c compared to thermometer reading. The heater is 4x the recommended wattage of the tank. So 20c on heater is 24c on thermometer in my tank


Not really less chance of cooking them, just the heater needs to be on longer using electric for longer costing you more if heater is too small


I go 100w per 100 litres


Thanks for your replies. So going by 100 watts/ 100 litres and 10 Gallons being about 38 litres, 100 watts is way too high. But I think you're also saying a too big heater will just work less hard than say a 50 watt in the same volume of water but would not pose a higher risk of cooking my fish if it got stuck on. Do I have that right?


I had a 200 watt heater in my 20 gal when one needed replaced (it was my spare) I got another 200 watt heater. I don't have to worry about cooking my fish and my heater doesn't have to work hard to keep the water at the temp I want it at.


Thanks for that.

I have read so many conflicting opinions about this -- I think the most compelling reason I have seen for 100w over 50 watt is build quality (typically better in 100 watt) but many of the resources I read were many years old.

Some sources say 100 W heater will work less hard and have less chance to fail, some say that if 100 W heater gets stuck on it has a greater chance of heating the water up so fast that I could never catch it in time compared to a 50 W in the same volume of water.

I have to admit I'm still torn.


I think its pretty impossible that the heater could get stuck in on position. The element would burn out its prob more likely that the heater malfunction would be that turns off for good!
If it did stay on then yes a heater underated is going to struggle to heat the tank as quickly but either underrated or overrated would prob heat the tank to an unsuitable temp within 24hours


Id go slighly over because
a. Turn on less often, for less time compared to underated.
b. Elec bills should be less, variable on many factors.

I have however put an massively underated heater in a way too big tank and found even constantly turned on that it couldnt reach a high temp. I guess it depends how underrated it is


Heaters do stick and cook tanks. Some of the higher quality heaters will shut off around 92 degrees because of a built in safety. You might look into a 75 watt Cobalt Neo Therm, it is really compact and will fit in a 10 nicely.


Theres a device turn a plug off attached to a thermostat you put in tank prob available in diy, plumbing or electronic stores that you could plug your heater into which would turn off when probe reaches desired temp


Thanks guys - I am going to weigh out everything and perhaps just suck it up and buy an $80 heater with built in safety. The difference between that and a $30 Eheim is great peace of mind. Okay, so I only paid $9 for the fish in the tank it's going in, but that's not the point! At least I wouldn't have to get up in the middle of the night (like now) and check his temp.

I like that Cobalt, fissh, thanks, just Googled it. Harder to find in Canada and hella pricey, but while I'd never really considered an electronic heater, the Cobalts look really nice and that one gets particularly good ratings (I like the internal shut off).

Rivermonster, yes, I know what you mean - like an external thermostat with a relay. I checked those out, too!

I guess any technology can and does fail. I ran the old 100w for 4 years with no issue (cheap Top Fin heater I had for a betta years ago) so I'm probably just being a worrywart, but I love these little guys and would hate to see anything happen to them that I could prevent.

Thanks again for the help, everyone! Much appreciated.


I have 2 different 50w heaters and was switching tanks around tonight. I had the eheim 50W in my 5 gallon betta tank. It always kept the temp just fine no issues at all. I just don't like the control dial. (I have bad arthritis in my hands, it was harder to maneuver). Anyways I was going to just keep that as my backup.

My other 50W is an Aqueon and was in my 10 gallon tank. My 10 gallon tank was a community tank kept at right around 75-76 degrees and I didn't have a single issue. Been using it for the last 5 months.

I went to put my betta into the 10 gallon with the Aqueon heater, but temp needed to be at 80, instead of 75. It could not get the heat past 78. I let it run for 5 hours. It only needed to climb 2 more degrees and it just couldn't do it. I put the eheim 50W heater in the 10g, and it was up to 80 within 30 minutes. It has been sitting at 80.2 for the last 3 hours without a single problem. Minor fluctuations here and there. Exactly as expected.

If I didn't have all these heaters, I think I would have bought a 100w one instead. I know what to buy next time for sure.

Now after all that, I have a 150w Aqueon Pro heater in my new 29 gallon tank. It has been up and running for 12 hours now and has kept the heat at 75.2 for the last 6 hours without a fluctuation. Gotta like that.


Thanks AllieSten. I like to hear about other people's experiences (not so much the fish cooking I've read elsewhere, but I digress )

Yes, this is for a betta so I'd like to be at 80. The 50w Marina in my 5 gallon is holding temperature well (and it's probably harder for it in a smaller tank since I'd assume ambient temperature would affect it more). Using the 3-4 watts per gallon I've read about, it's nearly as overpowered for the 5 gallon as a 100w would be in a 10g.

Interesting the difference you had between the two 50w heaters, even. I think if my LFS carried the Eheim in a 75w, I'd go for it but I'm encouraged to hear that you would opt for the 100w because I think that's all there is locally and I'd rather not wait until Tuesday to have one shipped. I have heard really great things about the Aqueon Pro, too.

Bah, I should have just got a 20 long for each of them and I wouldn't be fretting so much. If I just got rid of a few things, they'd fit. Hmmm.

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