Sick Black Molly

A sick black molly and an overstocked aquarium.

Q. Yesterday I added two black mollies to my aquarium. Right now one of the black mollies is at the bottom of the aquarium with a blue discoloration and growth around its gills and mouth. It is also breathing rapidly. I just upgraded my 10-gallon aquarium to a brand new 20-gallon. I moved all my old substrate, water and filters to this new aquarium.

I did an ammonia test and found only trace amounts of ammonia. I have two goldfish, two black mollies, one silver molly and several other silver fish (I’m not sure what they are). What is wrong with my black molly? Will my aquarium be all right and will the other fish get sick?
East Syracuse, New York

A. I see some potential problems with your fish aquarium and I am going to address each one separately. To answer the first part of your question about the ill black molly: I think chances are the blue growth or discoloration you are seeing may be a fungal infection. Extreme stress from unstable water parameters can cause fungal infections and from the symptoms you described it is my guess that bacterial fungus has made your tropical fish sick. Treating fungal infections can be very difficult and the most effective way to handle these is with an antibiotic.

Treating a fungal infection requires close evaluation and several steps. Bacteria may be present and the fungus you are seeing is secondary to a bacterial infection. Also, the initial culprit may have been a parasite and the fungal infection is secondary to a parasitic infection. Normally just treating a tropical fish with an anti-fungus agent is ineffective and to truly help the animal the aquarist must attack potentially degrading water quality, bacteria or parasitic infection and finally the fungus itself. Sometimes feeding garlic soaked fish foods in addition to superior water quality can clear these conditions up rather than creating a chemical soup of antibiotics and other medications.

Anytime a medication is employed in the fish aquarium it is best done in a quarantine aquarium. Most antibiotics as well as anti-fungal or parasite drugs kill beneficial bacteria and can stress healthy tropical fish. I prefer to attack the problem with diet and a healthy environment before trying to play doctor with medications. That said, deciding which antibiotic is right can be a major undertaking and using an antibiotic on the whole aquarium will often make matters worse. It’s often a better pursuit to correct the source of the fish’s stress rather than concoct a chemical soup of antibiotics trying to cure it.

It sounds to me like your bacteria bed may not be suited for the tropical fish load in your aquarium. Twenty gallons makes for a tight squeeze for two goldfish, two black mollies, and several other fish. Goldfish require not only cooler water than some of your other tropical fish but larger aquariums. The thought that a goldfish can survive in a bowl or small aquarium is a common misconception as goldfish eat a lot and create lots of waste (not to mention the fact that they get very large). I am sure when you made the transition from your old aquarium to the new aquarium that large colonies of bacteria died off. Even though you moved the substrate and water filters it can still take an aquarium several weeks to re-cycle once it has been moved.

It sounds like your ammonia test did show some trace elements of ammonia in your water. While it’s good that these amounts were not off the charts, chances are your fish aquarium is basically restarting the nitrogen cycle. It is likely that you will, in coming days, see drops and rises in both nitrite and ammonia. Finally you should see the nitrite and ammonia drop to 0 and the nitrates even out. Monitoring nitrate, nitrite and ammonia will be crucial for the next several weeks. At this point, since you already have tropical fish in the new aquarium, water changes will be the only way to dilute the rising ammonia and nitrite. I would recommend testing your ammonia as you have been, and also nitrite, and administering water changes of 25 percent of your aquarium’s water volume as needed. There are several products on the market that claim to speed up the water cycle or absorb ammonia and nitrite. I cannot attest to how any of these work, but they may prove helpful in your situation. Soon you will notice that the parameters causing problems (the nitrite and ammonia) will even out and your fish aquarium should be cycled. I will say that I wouldn’t recommend adding any more tropical fish to the aquarium.

I’m sorry I can’t offer more advice on the molly that has fallen ill. I honestly think at this stage adding anything that is intended to fight off a fungal infection would do more harm for the overall aquarium than good. If you have a small hospital or quarantine aquarium, however you could attempt to treat the infection there.

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Fish · Reef Tanks