my heater just doesn't seem to be keeping my tank at a stable temp. i would like it to be around 26 degrees but it drops to around 24 on cold nights. i thought it might be that the heater was not big enough for my tank (150 watt heater, 150L tank) but when the thermometer is low the thermostat light isn't even on.

my thinking is either the heater thermostat is faulty or that there is not enough water movement around the heater (there is a lot of driftwood along the back of my tank).

any ideas?

I had a heater that was doing the same, eventually it wouldn't turn on at all, i ended up getting so annoyed with it that i smashed it open and found that half of the bi-metal thermostat had corroded away.

it may be your thermometer thats dodgy a lot of ones for aquariums are very inaccurate

The heater is to low a wattage. You should have something about 200watt. That's what I use in my 150L and I've never had any problems. Yet when I had a heater that was 150watt I had the same issue as you.

Have your fish shown any signs of stress caused by this 2 deg temperature variance ?

If not.... Why worry.

My tanks drop way more than that every week when I do cold water changes. If anything the fish love it.

It even stimulates many species to breed.

what are the facts?

what brand of heater?

What is the room temp at night?

is it in a garage or particularly cold room?

do you have a second thermometer to check the temp?

@smidey it's a Fluval tronic 150 watt. it says up to 180L and my actual water volume is around 140L.

Don't have another thermometer, i should get one, whats a good brand?

Don't know the temp of the room but i wouldn't expect to be too cold, it's in my lounge.

@kiwiplymouth my fish seem fine but i'm worried about things like whitespot in the long term and will the heater die all together.

then i would say the weak link is the thermometer, i would be surprised if the temp is fine.

I have a couple of glass thermometers and they are not as accurate as the digital temperature gauge I had, often out by a couple of degrees. I always presumed the expensive digital one was correct. Have never tried the aquarium digital thermometers however.

get a jager heater, keep the temp right

a 150W jager heater should be fine in a 150L tank. That's what i run in my 170L tank and the temp is steady at 26oC.

Very pleased with the jagers.