Yesterday I noticed that my Lake Kutubu rainbowfish was not her usual self, that is when I put in the flake, she didn't shoot straight up and grab as much of the food as possible before the other fish could get it. She also allowed other fish into her territory and only chased them away half-heartedly. Close examination showed no obvious sign of disease. However, when I checked the temperature it was down to 18.5, i.e. room temperature. It didn't take long to find that the heater wasn't working. Fortunately, I keep a couple of spares and on replacing the heater she was back to normal well before the temperature was up to its usual 23.
Several lessons to be learned here :
1. Always keep spare key equipment. I have a couple of heaters, plus a filter which runs as an extra in one of my larger tanks and so is already cycled.
2. Check the temperature regularly, that doesn't necessarily mean a thermometer in every tank, I find that just putting the flat of my hand on the front glass every time I go to switch on/off lights or to feed them is a good indicator. In the present case, no disaster happened but had the tank been one of those in the unheated spare room, where the temperature was 12 degrees, it might have been very different.
3. Observe your fish carefully and note even small changes of behaviour, don't leave it until too late. I reckon fish behaviour is a better indicator of overall tank health than chemical tests.
4. The Zebra danios and Ruby barbs in the tank seemed perfectly happy at the lower temperature with no change in activity or behaviour which supports my theory that many (not all) fish can live just as well at a considerably lower temperature than the 25 or 26 that is often quoted as “normal”. Don't waste expensive electricity for nothing. The Zebras are already scheduled to be moved to another tank which is set to 20 degrees where they will share with barbs that are also happy at this level.