Hi Dayak.
A 300watt heater is really only rated for a 75 gallon tank.
Using that on a bigger tank the heater will be kicking in too much putting stress on it, which will likely cause it to fail.
IMO, it is always best to find the wattage rated for your tank, then go with a slightly higher rating. And try to get 2. (for example, if your tank needs 500watts, get 2 x 250watts connected to the one thermostat)
A 300watt heater is really only rated for a 75 gallon tank.
Using that on a bigger tank the heater will be kicking in too much putting stress on it, which will likely cause it to fail.
IMO, it is always best to find the wattage rated for your tank, then go with a slightly higher rating. And try to get 2. (for example, if your tank needs 500watts, get 2 x 250watts connected to the one thermostat)