Show Me the Color
Color changes in goldfish may be just that, or perhaps a sign of disease.
Stephen M. Meyer
Q. I have a 20-gallon long goldfish tank with two fantails, four comets and a black moor. The black moor has lost the covers from its gills and you can see the red gills inside. It has started turning lighter on the belly, but seems to be active and eats well. The skin around its eyes has become pale. The white comet has developed a few orange scales, and one of the gold fantails has one or two white scales.
They are getting big for their tank, but it has plenty of filtration. The water evaporates about an inch or 2 a week. I do 40-percent water changes every other week. I have treated the tank with Aquarisol, but see no results. This tank has been running about a year. What is causing the discoloration of the fish?
A. As regular readers know, I don't believe in disease diagnosis without seeing the fish. So many fish health problems — real and imagined — produce similar signs. Diagnosis is tricky enough when examining the animal directly. With only a sketchy description to work with the task is little more than pure guess work. So let's just work through the logic of problem.
Taken by themselves, the few orange scales on the comet and few white scales on the fantails, and the lightening of the moor's belly would not be any reason to think "disease." As goldfish mature they often experience color changes. This is especially true of typical hobbyist goldfish, in which traits are not well fixed. Multicolor fantails, for example, will gain and lose color and patterns several times over the course of a few years.
Although expensive "show" goldfish are (usually) bred to "fix" color more reliably, they too may change color with age. Even the best quality moors will lose their velvety black color — lightening to a dull bronze — if not exposed to bright light consistently.
That said, color change can be an indicator of a fish health problem. Poor water quality, in particular, can cause goldfish to grow paler as their health declines. Even low levels of residual chlorine in the water can "bleach" goldfish before eventually killing them. The loss of your moor's gill covers, however, is a sign of a very serious fish health problem.
I would begin by checking the water quality parameters. You may think you have adequate filtration, but how do you know? Assuming your goldfish are average size, you have far too many fish in that 20-gallon tank. Did you test for ammonia, nitrite and pH? Is there chlorine or chloramine in your water?
Perhaps one or more of your comets is overly aggressive. Crowding can cause aggressive fish to attack tankmates that are lower in the pecking order. Your moor's gill loss could be a sign that it is a victim (eye loss is also a sign of this kind of victimization).
I doubt the problem is parasites because you would certainly have seen white/gray specks, opal bumps, a milky gloss or some other unusual feature. I'm not surprised the Aquarisol didn't "work" — there was nothing for it to do.
I would reduce the fish load to no more than two fantails, or get a 55-gallon tank. Comets actually do better in a backyard pond anyway.