The general rule for heater size is 5 watts per gallon, but lots of things to consider before you get a new one. Most heaters are designed to raise tank temperature only a few degrees above room temp. Keeping tank in a cooler area, you would want to bump up in heater wattage above the general rule to compensate. I have several Jäger heaters, and a few marineland visitherms and have not had any problems with them. Depending on your tank setup, if your using a canister filter consider using an inline heater, they are supposed to be more efficient and less clutter in the tank. Get the best you can afford, it's worth the investment.