The Planted Tank Forum banner

1 - 14 of 14 Posts

·
Registered
Joined
·
733 Posts
Discussion Starter · #1 ·
Hello all, this is probably a silly questions but...

I have an aquarium thermometer that consistently says the water temperature is higher than what my water heater thinks. For example, if my heater is set to heat to 82F, the thermometer says the water is around 88F. But my heater is 'on' and heating when the thermometer says this. So I'm confused; because I thought the heater only heats when it's not the right temperature and turns off when it is.

I have a JW Fusion thermometer that sits in the tank using a magnet, and an aquatop 100watt heater. It's a 20L tank.

Do I need a new thermometer? Should I trust the thermometer? Is the heater wrong and I should set it lower than what I actually want?

Any help appreciated!
 

·
Registered
Joined
·
1,498 Posts
I had this same problem when I bought a Aqueon 300w for my 75gal. I had the heater set at 79 and the water temp stayed consistantly 82.
Took out the 300w put in the 250w set it to 79 and it stayed at 79.

Maybe you have to much of a heater.

also I even set the 300W to 70 and guess what IT stayed at 80.
 

·
Registered
Joined
·
5,370 Posts
I like the old style floating thermometers, but it really doesn't matter what the heater is set on as long as you know the water temp. Most thermometers use mercury or metals to get a temp, so the magnet could throw off the reading. It wouldn't hurt to pick up a new thermometer made without a magnet or atleast a different style, atleast it would be a third opinion toward the correct temp.

Is this it
 

·
Registered
Joined
·
5,370 Posts
BTW that 's a really high temp and most tanks would be 78 or 80 degrees so why is your heater on, if it's to check the thermometer it could take several days for the water temp to settle after turning on the heater.

I would also trust the thermometer, most heater temp markings are just a guide, so turn it down to 76 or off.
 

·
Registered
Joined
·
1,956 Posts
If I could stick my finger in the water I could tell you if it was close to 88°. lol

Maybe drop by Walmart and buy a new thermometer for less than $2.

But I'd guess the heater temp is off and you can adjust it down to compensate. 100w heater is OK unless it gets stuck on since it wouldn't give you much time to find it out.
 

·
Registered
Joined
·
733 Posts
Discussion Starter · #6 ·
I like the old style floating thermometers, but it really doesn't matter what the heater is set on as long as you know the water temp. Most thermometers use mercury or metals to get a temp, so the magnet could throw off the reading. It wouldn't hurt to pick up a new thermometer made without a magnet or atleast a different style, atleast it would be a third opinion toward the correct temp.

Is this it
Yes that is the thermometer I have. You really think the magnet might throw off the reading?
 

·
Registered
Joined
·
1,746 Posts
A magnet can't affect a mercury thermometer, and virtually any thermometer sold to the public in the last 15-20 years for purposes other than scientific or medical use contains any one of a half dozen organic compounds (ethanol, toluene, etc) and some red dye. Unless the line in the middle of the thermometer is silver colored, it doesn't contain mercury.

In any case, you have two items that are disagreeing, the thermostat from your heater and the thermometer. There is no way to tell with these two alone which is correct. Do you have any other thermometers in the house that you might use to check the others? Ideally it would match one of them, but honestly, it will probably read a third value anyway.

If you decide to buy a new thermometer, look at all of them on the shelf and pick the one with the reading closest to the mean of the lot. (Not my suggestion, read it on here recently but it's a really good idea.) I'd be willing to bet that the thermometer that you have is off, as 88 degrees is pretty hot to keep a fish tank without something suffering.
 

·
Registered
Joined
·
637 Posts
The temperature markings on heaters are nearly always 'more like guidelines anyway'. I would trust the thermometer more and turn the heater down.

Like other people have suggested I would go out and buy another thermometer and see what it says, since they're cheap anyway. The first thermometer I bought didn't work at all for some reason; it was just the standard glass floating thermometer that everyone has, but for some reason it always said that the temp was like 75 even when I put it in the fridge!
 

·
Registered
Joined
·
733 Posts
Discussion Starter · #9 ·
A magnet can't affect a mercury thermometer, and virtually any thermometer sold to the public in the last 15-20 years for purposes other than scientific or medical use contains any one of a half dozen organic compounds (ethanol, toluene, etc) and some red dye. Unless the line in the middle of the thermometer is silver colored, it doesn't contain mercury.

In any case, you have two items that are disagreeing, the thermostat from your heater and the thermometer. There is no way to tell with these two alone which is correct. Do you have any other thermometers in the house that you might use to check the others? Ideally it would match one of them, but honestly, it will probably read a third value anyway.

If you decide to buy a new thermometer, look at all of them on the shelf and pick the one with the reading closest to the mean of the lot. (Not my suggestion, read it on here recently but it's a really good idea.) I'd be willing to bet that the thermometer that you have is off, as 88 degrees is pretty hot to keep a fish tank without something suffering.
Hmm, alright. Yea, all the fish seem fine. And the water doesn't feel warm to me... though I can't be an accurate judge of temp
 

·
Premium Member
Joined
·
1,533 Posts
I almost always have varying tank temps from what my heater says. I don't even bother using the heater numbers except as a guide when I first set it up. Then I just adjust it given what the thermometer says.
 

·
Registered
Joined
·
733 Posts
Discussion Starter · #11 ·
Alright, so I got a second thermometer ("stainless steel thermometer"). It reads 86F while the old one still reads 88F... So should I trust this new one?
 

·
Premium Member
Joined
·
1,533 Posts
Alright, so I got a second thermometer ("stainless steel thermometer"). It reads 86F while the old one still reads 88F... So should I trust this new one?
Get some crushed ice and a little bit of water to make a slushy mixture and test the thermometers, whichever reads 32 degrees is accurate. If it goes high enough (not likely) you can boil some water and test to see which one gets to 212. (Degrees F)
 

·
Registered
Joined
·
637 Posts
I would trust the two thermometers to tell you that your heater is set incorrectly. 2 degrees F doesn't make a whole lot of difference, but they are much closer together than the 80 that the heater thinks it's at. Whatever the case, I would turn the heater down. Unless you're keeping dicus your fish are probably boiling (and even for discus it's high).
 

·
Registered
Joined
·
733 Posts
Discussion Starter · #14 ·
Get some crushed ice and a little bit of water to make a slushy mixture and test the thermometers, whichever reads 32 degrees is accurate. If it goes high enough (not likely) you can boil some water and test to see which one gets to 212. (Degrees F)
I placed both thermometers in the fridge for about 30mins. The fusion one read 46F and the stainless steel read 38F. I'm guessing there should not be such a huge discrepancy. How cold are fridges usually set to? I saw online it's usually 40F or below. So I'm thinking the stainless steel therm is the more accurate? Thoughts?
 
1 - 14 of 14 Posts
Top