Is it ok to use a 100watt heater (that says "for 100 - 150 litres) in 40-50litres of water?

I don't wanna boil my fishies

If not, let me know so I can buy a 50W heater asap today for medicating. I wonder why they give a water volume range in heaters when I thought thermostats will regulate the temperature anyway :\

Not all heaters have thermostats.

I would just be careful if you get a larger heater to watch the temp carefully & have a thermometer in the tank. I wouldn't just trust the thermostat setting to be correct (none of mine are)

Good Luck!

With heaters you want 5 watts per US gallon. And it is recommended to
have a thermometer in the tank.