It keeps my 30gal about 76.4 degrees F. Definitely give it time in the water unplugged to equalize the temp before plugging it in or transferring it into a bucket of room temp water if you need to use it to heat exchange water. I think i nearly broke mine by pulling it straight out of my tank and putting it into a cool bucket of saltwater, while it was plugged in. It screwed it up for a while and the sand inside can shift around now exposing heating elements, but, it still works... I did what it explicitly says NOT to do in the instructions, so this was my fault.