I purchased a 100 watt Aqueon submersible heater for my 20 gallon. It is upright in the tank, with the dial out of the water, above the minimum water line. The first time I turned it on it made this weird knocking/clicking noise when it was heating the water. It freaked me out, so I unplugged it.
When I plugged it back in a few days later it continued to make the knocking noise, and now has condensation on the inside, like when you leave a water bottle in a hot car.
I assume that having water inside the heater is dangerous. Should I replace it? If so, what with?