I just added a second Hydor 300w inline water heater to my canister filters on my 75g. a few days ago. Strangely, over the course of a few days, the water temp has risen from 80, where the heaters are set (for African cichlids), to 83.5. Yesterday I unplugged the older Hydor, thinking it might be acting up. But this morning the temp had still risen two tenths from the night before. So I unplugged the second newer heater four hours ago. The room temp is about 80.
First, how do I safely get the temp of the tank back to 80? I need to do a water change today but don't want to shock the fish.
Second, what else could be causing the temp rise?
The only other change was adding some crushed coral to my filters about 5 days ago.
The heaters are unplugged right now while I wait for the temp to drop back down.
They were both set to 80 degrees. They have internal thermostats that are supposed to activate them when the water temp gets lower than the set temp. And, obviously, they're supposed to not heat water that is already at 80 degrees.
What are they good for?
Should I just switch them off?
If it's 5 to 9 watts, it's good for green water or cloudy water. It will not kill pathogens or parasites. I would switch both off for now, and see if your temps drop. I use a 15 watt UV in my tank but I like my temps high because of my discus.