IMO the main problem would be when you turn them off (temperature fluctuation). You can't keep them on 24/7, and when you do turn them off, the water usually drops down because at night, it gets colder. That would be the main thing.
hey is that cheap tank heater at wal-mart any good? Like it is the right size that i need and i have read a lot of mixed reviews on it, Sooooo i am not sure if it is worth buying or not. I am currently using desk lamps to heat my water and it keeps the water at 78, but i'd rather have proper tank heaters haha but all the ones i have seen at petco and petsmart are way to big for what i need