Heater issues

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Aquarium Advice Newbie
Sep 25, 2023
Hi I’ve just recently got myself a 70litre tank that come with a 150w heater. My tanks heater reads 4 degrees higher than the actual temperature it heats the tank too it was set to 26 degrees but was reading at 22 but my tank dropped temp from 22 to 19 over night so I bumped up the heater to 28 degrees for the tank to read 24 but it still dropped 3 degrees overnight to read 21 so I have 2 questions
1 do I need to replace my old heater with a newer one or is it normal for the heater to read different to the actual temp
2 will a new heater keep the temp stable even over night when it gets cold or is it normal to slowly drop 2-3 degrees over night to slowly get back to temp throughout the day
1. Yes, its normal for a heaters to not heat to the exact temperture you set it to. Adjust the temperature setting until you get the water at the temperature you want. If you need to set it to 28 to get a water temperature of 24 thats not a problem.

2. Depends on how good the thermastat is on the heater. Ive had heaters that kept the water consistent. Ive had heaters that allow the temperature to drop a degree or 2 overnight when its cold, and then warm up again during the day as the heating comes on. Never caused me a problem but it depends on how much that temperature range is. 3 degrees C might be a bit much.

What you see might be an issue with not having enough flow around the heater. You want the water to warm up and dissipate around the tank. If its just warming up the water around the heater and then not circulating, the water temperature around the tank wont get the benefit and you will see these temperature fluctuations when the room temperature drops. I like the heater next to the filter intake so that the water warms up as it enters the filter and the filter output circulates that water throughout the tank.

Another issue is 150W is a big heater for 70 litres. You typically want 1W/litre plus a little bit. So a 75W heater is good for 70 litres. 100W if your room temperature is particularly low so it has the power to raise the water temperature that little bit more.

An oversized heater will heat the water quicker then turn off, coming back on when the temperature drops. So its constantly turning on and off. The heating element wont be on so much so this preserves the heating element, but the thermostat/ switch will wear out quicker because its turning on/ off more often.

An undersized heater will take longer to heat the water, so the heating element is on more and will wear quicker, but the thermostat/ switch will be preserved as its not turning on/ off as often.

You will find advocates of both using oversized and undersized heaters, but IMO having the right size heater is best.

Another issue with an oversized heater is if the thermostat/ switch fails in the "on" position it will more quickly heat up the water and you could overheat your fish before you notice a problem.

You ask do you need to get a "new" heater. Did you buy your aquariun set up used? Ive never had a heater last much more than a couple of years, so if its a used heater i would probably replace it as it might not last much longer anyway depending on how old it is.

What brand heater is it?
It’s an Aqua one glass heater and yes the heater came with the new tank so unsure of its age.
We have quite a cold room temperature in our house, and can get quite cold during the night especially when we haven't had the fire going so would you still recommend the right sized heater for the tank or one a little bigger? The fish don’t seem to be phased by the 3 degree drop during the night but I’d prefer for it to be more stable.

I also don’t have the heater right beside the filter intake but it is on the same side/wall of it should I move it closer? And do I also need to insulate my filter pipes for them to hold the heat better? Or should this all be fine

You are also right about the heater turning on and off all the time whether the tank is at the right temp or not.
If your low room temperature gets down as low as 10 or 12C and you need to raise the water temperature to 24C then you would need the 150W on your 70 litre tank. If you live in a poorly insulated house in a country that has very bad winters that might be conceivable. Our late 1980s built house in Derbyshire UK gets down to about 16C overnight in the winter if the temperature outside is sub-zero and then warms up to room temperature of 19C when the heating comes on in the morning. If i vacate the house for a few weeks and turn off the heating then it will drop as low as 12C.

A 75W heater should be good to raise the temperature about 8C in your tank. A 100W heater should work for about 10C.

Ive never used an aquaone heater, although id say its a mid range brand. You could spend money on a better brand and see no difference. I used to really like Fluval M series heaters, but dont seem to be able to get them locally anymore. Im currently using JBL heaters which are OK and they seem to be the best i can get locally from the ones ive tried.

Id sit the heater right next to the filter intake.

Anything you can do to reduce heat loss will help maintain temperature stabilty and reduce the need for heating. Ive never considered pipe lagging before and never heard of anyone doing it, but yes it would help. If you didnt also insulate the sides and back of the tank i would say the benefits of pipe lagging would be minimal though, and all that would start to be visually intrusive IMO. But some people do put sheets of insulating board on the sides and back of tanks, and insulate the lid, especially if they keep the aquarium in an unheated space where the temperature can fluctuate.

I cant really tell you that you have to replace the heater. An oversize heater will work, an undersize heater will work as long as its not too undersized. The wrong size heater will wear out quicker and there are risks involved once the heater fails. I cant tell you how risk averse you need to be. 2 x 50W heaters will be better than 1 x 75W heater. 2 will give you better heat distribution throughout the tank and give redundancy should 1 fail. But its a 70 litre tank and i think 2 heaters is going a bit far. If you really want to go all in a seperate thermostat controller by someone like Inkbird is belt and braces, but again its a 70 litre tank.
What sort of thermometer are you using?
If you are using a glass thermometer that floats around in the water, that should give you the correct water temperature assuming the thermometer isn't out. When buying thermometers, check a group of them and get one that has the same temperature as the majority of the others. Sometimes one or two will have different readings and these are not good.

If you are using a digital stick on thermometer that sits on the outside of the aquarium, they can give you incorrect readings because they pick up the cold air in the room and it counters the warmth coming through the glass.


A 150 watt heater should be ample for a 70 litre tank. If it can't hold the temperature at whatever it's set on, it could be faulty or the tank is in a really cold room and the heater can't warm it up. If the tank is in a really cold room, the heater should be on almost continuously and there should be a small red light that is on whenever the heater is warming the water. If the heater light is turning off and then coming back on 30-60 seconds later, it is working correctly and the thermostat inside the heater is most likely faulty.

If it's a secondhand heater, just use it until you can replace it, assuming it needs replacing.


You can insulate the aquarium to help the heater work more efficiently. Stick sheets of 1-2 inch thick polystyrene foam on the base, back and sides of the tank. This will insulate most of the tank and the heater should work better.

Make sure you have a coverglass on the tank to trap heat. Use 4, 5 or 6mm thick glass because it is less likely to chip or crack and holds temperature better than the thinner glass (2 or 3mm thick) usually sold in pet shops.
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