can you put a 300 watt heater in a 45 litre tank

Should be fine but set it a couple of degrease lower as it will heat your tank very quickly...

That wont be a problem. Strangely enough, the bigger wattage the heater the cheaper it is to heat your tank. Adjust the heater so it gets your tank to the right temp, you should have some sort of thermostat so you know what temp the tank is, dont rely on the temp markings on the heater

ok thanks delboy, i wasnt shore about it, incase it fried the fish. i will put the thermostat in there.

When i said you need some sort of thermostat i meant thermometer

Tank heaters work in a similar way to your heating system in your house. When they get to the set temp they turn off. The reason you need a separate thermometer is that sometimes the temp markings on the heaters are inaccurate!

right it is thermometer, i just repeated what you said

the thermometer is in there

i know you knew whay i meant, was just making myself clear to anyone else reading