Temperature Changes In The Tank , What Are The Causes ?

Tank water is indeed affected by room temperature. The usual procedure is to set the heater for the temp you want and just leave it. Heaters have thermostats. If the water temp drops, the heater switches on. In a heatwave where the water gets warmer than the heater is set for, the heater does not switch on. You don't need to keep altering the heater.
Think of it like your central heating. In winter, it comes on till the house has warmed up, then turns off till the temp drops, then turns on again. In summer, it never turns on. You can leave the thermostat at the same setting all year round.

Heater dials are rarely accurate. They are best set using a thermometer and ignoring the number on the dial. This needs to be done when we are not in the middle of a heatwave. Turn the dial to a setting that is lower than the temp you want (if there are fish in the tank, not much lower) and allow the water temp to adjust. When the heater light comes on, look at the thermometer. If the temp is too low, turn the heater dial up one notch and wait till the temp has settled, then look at the thermometer. repeat until you have the temp you are aiming for.
Some treatments require you to turn the temp higher. Always make a note of the heater dial setting before doing this so you know where to set if after treatment has finished.


The stick on the outside thermometers are the least reliable as they are affected by air temp as well as water temp. Go by one that sits inside the tank under water or use a digital thermometer (the kind with a probe that goes in the water and a readout that sits outside the tank).

I didn't realise the heater switched on and off ! Everything you've both said makes sense . I just find the glass thermometer really hard to read to an exact temperature . The shop told me as long as it's I. The green zone it ok for tropical fish, but how would this be do to cater to different fishes temperature needs ?
I like the one on the outside as its easier to read but can fluctuate , obviously due to room temps likes sue has said .
I will set it like sue suggested .
Thanks
Ally

I didn't realise the heater switched on and off ! Everything you've both said makes sense . I just find the glass thermometer really hard to read to an exact temperature . The shop told me as long as it's I. The green zone it ok

Agree with Sue, it is preferable to have the temperature set "Just so" rather than anywhere in the green zone... I think I may be able to assist with your thermometer readings...



The thermometer on the right is one I got with a bunch of stuff a lady was selling on gumtree. You can see the difference from the more common sized thermometer. I popped it in and left it for a while and there was less than 1 degree celcius in difference to the readout.

I don't need it, so you might as well have it 

If you would like it, PM Me your address and I'll pop it in the post tomorrow 

With a 22litre tank then the water will heat up and cool down faster than, say, a 120litre tank. So a big tank, on a very hot day, won't show much change where as yours will. I have a 55litre tank and I don't use a heater at all, I just let it follow room temp, and all my fish (except for my new Otto's) are over three years old and doing fine. I've never seen the tank above 24ºC or below 18ºC.