I went to feed my fish this week and the loaches in the 15g aquarium didn't immediately come out as they usually do. I checked the water temperature and it was 70° instead of 80°! The heater had died after only two months and I hadn't noticed, my bad!
I had a spare new Fluval heater which I installed immediately, but to help my loaches I removed a gallon of water and added another gallon of warmer water from the faucet. It raised the tank temperature a little before the heater could. It's now holding at 77-78°, that will work. The loaches are active again.
The heater in the 29g is keeping the temperature at 75° instead of 78°. The Kuhlis are still active, so I think I can hold off switching heaters, for now.
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The heater in the 5.5g wasn't working either, are malfunctioning heaters contagious?!?
The temperature in the 5.5g was at 68° (room temperature). I put in a spare used Topfin 50watt heater, and temperature went up to 80°. I dialed the heater back to the minimum and it appeared to be stuck, water hit 90°. That heater went in the trash, and I moved my Red Cherry shrimp over to the 15g tank while I troubleshooted the heater situation. I dug out another used heater and installed it on the 5.5 aquarium. It has since brought the temperature up to 78° and if it can hold it there I will move some shrimp back into the 5.5g, as they breed more successfully in a species-only environment.
I ordered two more heaters, on sale, but a good trusted brand, Fluval. They'll be on hand the next time I have heater issues.