Two weeks ago, I noticed a sore on the tail of one of my goldfish. I didn’t know the cause, but suspected a fungus. I snapped a couple of pictures of the sick fish and went to the pet store.

As luck would have it, the resident goldfish expert was there. He said it looked like damage from another fish–something other than a goldfish as they’re not the least bit scrappy. Maude, the swordtail, may have been in a mood. Whatever the cause, he suggested treating the tank with an antibiotic.

I expressed my desire for panda fantails and my reluctance to put them outside or in the tank where others have perished. He asked a few questions and quickly identified the problem: too many fish.

WTF?

I have a 29-gallon tank with six fish: four goldfish, a swordtail and a tiny catfish. That’s like five gallons a fish (4.83 to be exact). Granted, there are lots of variables, but one fish per gallon has been my rule of thumb for ages.

The rule works well enough for smallish tropical fish. The goldfish are huge, and it turns out, produce MASSIVE amounts of waste. The high concentration of nitrogen is potentially toxic, especially for smaller and/or more vulnerable fish.

Turns out, my 29 gallon tank is big enough for exactly one goldfish to live comfortably. Housing the six I have now would require at least 150 gallons. So much for the panda fantails I covet.

I immediately reduced how much food they were getting and moved the biggest fish from the tank to the pond, leaving three goldfish in each location. The pond holds maybe 70 gallons and is doing fine. I’m having to replace a third to half the water from the tank every few days to avoid toxic water quality.

My goldfish era is rapidly drawing to a close. A local pet store accepts “surrendered” fish–a little favor that’s likely to net them a couple of hundred bucks. They’re going back sometime this week.

On the plus side, I no longer need a bigger aquarium. The one I have is more than big enough for the guppies I’ve dreamed about since childhood. I’ve had millions of guppies, but never a tank totally devoted to them.

Pet stores tend to put several kinds of guppies together. Instead of looking like their beautiful parents, the mixed offspring revert to the basic guppy found in the wild. I want purebreds. The pet store that’s re-homing my goldfish is happy to special order breeding trios (two females and a male) of the same variety for me.

Transitioning from goldfish to guppies means raising the water temperature. Before hooking up the heater that came with the tank, I need to find a thermometer I can read. As always, I’ll keep you posted.