Greece is a pretty warm climate I think. You may not need heaters but definitely get a thermometer for each tank. That way you can see if the temps are holding steady at 76 or above. If not, then turn on the lamps. Bettas do like some time without lamps, just like we do, so be aware of that.
Temps that plunge then rise often can bring on ich and fin rot so monitor those temps!
Bettas can live under "difficult situations" but better to keep them comfortable and thriving, don't you agree?
PS: don't use aquarium salt for more than 10 days.