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Ok I just did a full tank cleaning (since one fish died in there and I don't think it was sick but best to be safe) and I put the tank heater in and the light keeps going on and off, on and off. And it isn't flickering it's either on, or off. And it either stays on for a bit or goes off for a bit. I usually keep the temp around 75-78 and after I filled it back up it was 84. So is it just doing this to keep the water a little bit warm instead of cooling down too quickly or do I have a short and need to get a new one?

on most heaters, the light goes on when the heater comes on.... usually the heater doesnt stay on constantly all the time or the water would get too hot.

it just kicks on when it needs to warm the water up

think of it like the heater in your house... its not constantly blowing hott air, it goes on and off

thats what the heater in your tank is doing too =)\
there is nothing wrong with your heater, its perfectly normal

What brand of heater is it? Whats the room temperature? Do you have a filter outlet nozzle directed at it? Did you leave the heater in the water for about 10 minutes before you plugged it in?

It's a tetra heater. The room temperature is 70. THe fliter is away from it and the waterfalls from the filter aren't hitting it. I did leave it in the water 15 minutes before I plugged it in.

If it is far away from it, then not enough water circulation reaching it and it's only heating the water around it. I once tried to heat my large tank with the filters off and the water was too cool. If the temps around the filter are incorrect, then it does sound like you need a replacement. I think their heaters have a 2-year warranty.

I take my heater out for the summer exactly for that reason. Even without the heater the water heats up eventually...

It is close enough but not so close that the water would vibrate it into having a short. And the two year warranty is only good if you have the sale receipt which I don't. So I went out and bought a new one today even though I think this one is still working as the water temperature is 75 (but I just don't trust the lights being on and off on and off) and luckily I did buy a new one because I helped a guy who just bought a red ear slider get his set up ready (he was feeding it goldfish.....and much more)

If you just changed the water and the water isn't uniformly warm, the light may go on and off when cooler water circulates around it. When I use heaters in cooler weather, one of my is constantly going on and off as well, until the water temp is fairly uniform.