i am not in europe otherwise i would have bought a 4000 watt automatic heater. Here they aren't available.No, the problem is that it gets down to the mid-40F range. That's the number that determines how much heater capacity you need; a thermostatically-controlled heater automatically turns off when temps rise above the set point, and then turns back on when they drop below.
Of course your fish become gradually more active as temperatures gradually grow warmer. That's actually not a good thing; a fish that barely eats because it is experiencing a temperature that is near the bottom of its acceptable range will have its metabolism slow back down if temperatures drop again...as they certainly will at some point as the winter progresses. So it stops eating again and its minimal metabolism will not digest the food in its stomach, which will simply decay. Dead fish.
I just saw your latest post, with a grocery list of warmwater fish you are considering, so I will bow out now. Good luck.
Edited: Now I just saw your newest post; you can't be serious. That is a handheld heater designed for kitchen use, to boil water for tea!
BTW the other fish have managed to pull through one month of harsh winter