Is it getting too hot or too cold? and how quickly, in 400L it should take a while to change temp.
try having a thermometer just outsude the tank and see if the temp varies the same in the room, if so your heater is not keeping the tank warmer than the room and you may need to just turn it up. or if it's getting to hot, ie the room is getting warmer thn you would like, either fork out for a chiler or a few frozen coke bottles of water in the top during peak heat will work,(sort of in 400L)
If however the fluctuations are hotter than the surrounding room, then your heater may be getting stuck.
Could your thermometer be broken?