My tank is only 25 litres, I want something that is basic, inexpensive. I've been looking at the Aqua One heater, it's 55 watts and heats a tank up to 50 litres, are they okay? They're on trade me for just under $20.

yep that would be good for your tank.

They are not as good as jaeger but a hell of a lot cheaper. I have some in small fry tanks and they are OK. The temperature you get is not what it is set at.

It will be fine for your tank.

Make sure to have a SEPERATE thermometer to make sure the heater is heating the water right, with my sometimes it overheats the tank slightly, e.g. Set at 23, thermometer reads 25, but that's all good.

For your tank it might overheat a little, since tank is small and heater is very large with respect to the size of your tank, so set the heater a bit below your desired temperature.

would i be better off with a 25 watt heater then?

25 is cutting it a bit fine I think. Smaller tanks have a larger surface area to water volume ratio, so lose heat easier than a larger tank. I have a 35 litre tank with a 55 watt heater, and last winter when it got really cold, the heater couldn't keep the tank up to temperature

I would still go with the 55W.

Even if it overheats a little you could always turn it down to adjust. 25W is too small and probably won't be enough.

AquaOne 55W sounds perfect to me for your setup.