Question
I have a volitans lionfish that’s been in my 200-gallon tank for over two years, and lately it has begun to act like it can’t see. At feeding times, it makes an effort to grab food that’s drifting by but usually misses—like it can tell that food is in the water but can’t see it well enough to get it in its mouth. Is there a way to tell if this fish is actually going blind versus having some other health problem? – Submitted by Sean Myles
Answer
Thanks for your question, Sean! Jay Hemdal addresses the issue of blindness in fish—specifically the subtle symptomatic differences between a fish that is going blind and one that is actually ill to the point of becoming moribund.
Here is that particular excerpt from his book, which I think will answer your question better than I can:
Blindness
A very common symptom reported by home aquarists is that one of their fish has become blind. This, more often than not, is a result of a fish becoming ill to the point that it is moribund (close to death) and is not just blind. Basically, a fish that bumps around the aquarium, runs into the tank sides, and ignores food may not be blind at all; it may just be dying.
A truly blind fish will behave as if it is night and may even show its nighttime coloration. The fish will swim very cautiously to avoid running into obstacles, and it will orient itself in the normal upright position. If food is added to the tank, it will attempt to seek it out, perhaps by moving its mouth along the bottom of the tank, snapping up any food that it may come in contact with.
It is important for the aquarist to be able to distinguish the subtle differences between these two problems, as a truly blind fish may live for many years given extra care, while a moribund fish will continue its health decline and soon die if the problem is not corrected.
Bright aquarium lighting is sometimes implicated in causing blindness, especially in lionfish (Pterois spp.). Only anecdotal reports of this are available, and since aquarium lighting is many times less intense than on tropical reefs, cause and effect cannot be linked. Feeding freshwater fish to marine predators (again, often lionfish) is also reported by home aquarists to cause blindness. In these cases, it is most likely that the predator has developed fatty liver disease and the fish has become moribund from that.
