Thermometers! Dang things are driving me crazy!


Okay so I turned 50 this year and I can't see the dang thermometers to read them so I bought a digital one and put the little wire in my tank and hung it on the outside like the instructions said. All I could see on the old one was that I was in the "green" which meant I was somewhere between 70 and 80 degrees.

So... the new one with the wire said I was at 81 degrees... What?! I didn't believe it but... it was so easy to read...

So... I decided to buy a couple more since we had a big sale this last weekend being a be holiday and all the online stores had big sales so I bought two more from two different manufacturers...

Now I have four thermometers on the same tank. All are within 5 inches of each other and this is what I got...

One is in the "green" I would guess about 78 degrees... Oh and my heater is set at 76 and it comes on now and then...
One says 81.1
One says 82.4
One says 83.6

Now what do I do... I really wanted to put the digital ones on different tanks... I have 8... so I would know what the temps are. Now I think I might as well dip a finger and guess.

Are all thermometers this inaccurate? If so... how do we tell what the temps are?

So frustrated


Find an accurate thermometer (the old-fashioned ones tend to be the most reliable ones) and see how much each of the digital ones deviates. Once you know how many degrees they're off, you can probably get a reasonably correct reading from them.

For my shrimp, I use a TDS meter (total dissolved solids) that also has a thermometer. You have to hold it into the water for a little while, but this is a thermometer I pretty much trust and use as base to evaluate my digital and stick-on thermometers. It's about 10 bucks at ebay.

Or buy a magnifying glass so you can read the other thermometer. That's what I use and need to look at my small fish and shrimp anyways. Getting older sucks!


come on now... a thermometer!... the technology for that is so old school I would think they couldn't screw it up. I mean think about it... we use them every day to take the temperature of people and we need it to be accurate to the tenth of a degree and... it is! Even for the cheap five dollar ones. So why are all these things off by so much? I even have two of the same brand that don't read the same! So what gives?


A couple of thoughts here (well, more than a couple, because I think too much):

First, you don't need your tank to be accurate within a degree. Any of those digital readings are close enough to each other to be useful.

Second, it appears that your tank is somewhere in the low 80s, and you really need it a little cooler than that. (You already know that, obviously, because you have your heater set to mid-70s, which brings me to my next point...)

Third, the calibration thermostats in aquarium heaters are notoriously unreliable, which is why we bother with separate thermometers anyway. The fact that your heater turns on occasionally doesn't mean that it's maintaining the temperature at the setting you have.

My advice would be to turn your heater down until any or all of the thermometers read somewhere around 77-79, and don't sweat it if they don't all agree.

I expect that the problems are caused by shoddy workmanship more than anything. The heater dial probably isn't lined up exactly as it should be, and the construction quality of any design of thermometer will affect its accuracy, even if the technology is supposedly foolproof. Even the old-school glass tube thermometers are only as accurate as their placement on the piece of cardboard that has the numbers on it.


Good morning,

I also use the inexpensive glass thermometers:

(cheaper than $5 too!)

I keep my thermometers in my filters so that it always has a flow of fresh water around it. IMO, it's more accurate than placing it in the tank itself. I'm sure you know heat rises so you may see a difference depending on the placement of the thermometer, at the top, in the middle or toward the bottom of the tank and how close it may be to a heater.

Usually I use 2 thermometers per tank and compare. As long as they are within 1 or 2 degrees of each other, then I do not worry about it. Also when adding new water, I pull the thermometer from the filter to make sure I can match the temperatures as closely as possible.

As inexpensive as these glass thermometers are, I usually buy 5 or 6 at a time and do my own comparisons.



This is good advice and I think I will take it. Thanks.

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