One Betta.I think 50w is the smallest heater you're likely to find; it would be fine for your nano.
What are you thinking of keeping in there?
ahh that's what I wanted to know, thanks for that. I'll get a 25w. I only see the interpet ones at pets at home and they're very long. Too long to go in the compartment in the back of the tank. I'll go for one on ebay. Thanks very much for the adviceThey are selling 25W heaters in pets at home now.
the problem with the higher wat heater is: it puts a lot of strain on the heater (turning on and off much more frequently) and the thermostat isn't as accurate (years abo, before small heaters came out, I had the smallest heater I could get at the time in a small tank, and even on its lowest setting, nearly cooked my fire mouths).
A 50w in theory would be fine, they switch off when they reach the desired temp. I would say you would be ok putting a 50w in there, but it would take up a fiar bit more space than some of the tiny 25W ones ive seen. Based on that theory, you could put a 300W in there if it was all you had.
I think the problems only begin if the heater fails. They normally fail in the "ON" position, and a 300W heater in a small tank will boil it in minutes. A 25W heater would give you more time to spot the problem before it damages any livestock, or anything else.
I would say if your happy with a 50w going in there, it wont be so much of a problem, but to safeguard against the above, its always better to match the heater to the tank. I think even Argos do a 25w heater now.