Treatment of Fish Disease in the Aquarium

12.1. Basics of Treatment of Fish Diseases

This article goes over some of the basics for medications:

  • Stock of medications
  • Medications and scaleless fish
  • Medications and beneficial bacteria
Etroplus maculatus - Red Chromide
Etroplus maculatus Red Chromide

Stock of Drugs

It is very important to be prepared. Fish disease can move fast, very fast!  A typical stock is:

  • MetroPlex (metronidazole or “metro’” for “stringy white poop” [“Hexamita”] and “internal parasites”)
  • Fenbendazole (for roundworms, camallanus)
  • Maracyn 2 (antibiotic for gram-negative bacteria: fin rot, ulcers, gill disease, mouth rot, saddleback and red spots).
  • Ich-X (for ich)
  • Sodium thiosulfate solution (chlorine remover)

This store of drugs is important since most pet stores no longer stock most of these drugs (or any useful fish medications). Many keep this stock in a refrigerator to prevent it from expiring. Note Ich-X should never be refrigerated.

Note also that some of these medications will probably become difficult to obtain in the future as the FDA is moving to prevent these medications to be sold without a prescription from a veterinarian. Note also that many of these medications are not currently available outside the United States. None of these medications are available in any form in Canada or most of Europe. England has some relief in the form of a law exempting medications for minor animals but most suppliers do not know about the exemption and still won’t send out medications to England. Australia and New Zealand have mixed and constantly changing regulations.

Note also that some enterprising vets in Ontario Canada have opened up a very useful website,, which gives a free analysis of your fish problem and then allows you to buy some very good medications for the problem. While this service can only prescribe drugs for Ontario residents I suspect there will be other services that will copy this elsewhere in Canada and possibly even in Europe.

Note also that leftover human medications such as antibiotics can be crushed and used for fish. Just be aware that the strength is typically much higher so a very small smidgen in food is called for.

Nannacara adoketa - Zebra Acara
Nannacara adoketa, zebra acara

Medications and “Scaleless” Fish

Note that almost all authors and experts on the aquarium caution about using medications with scale-less fish such as plecos, corydoras and loaches. But this “sensitivity” is not supported anywhere in the scientific literature. All the millions of dollars of aquaculture research done on catfish and medications has shown no increased sensitivity to medications.

This myth probably started when someone somewhere in the distant past thought that medications are absorbed through the skin of a fish. The next step in this line of logic is that because scaled fish have thicker skins, they will resist overdoses of medications better than a fish without scales. And the final step in this line of logic is that scaleless fishes need lower doses of medications.

Cynotilapia sp Hara
Cynotilapia Sp Hara

The problem is that medications and all other chemicals in the water are NOT absorbed through the scales and skin of the fish. Chemicals are absorbed through the gills if they are adsorbed at all. So, scaleless fishes have identical sensitivities to medications as do scaled fish. Medications at the recommended strength will not kill scaleless fish.

Lots of full-strength meds like copper are used in raising catfish for food and they don’t die, even though they are “scaleless”. I’ve used full-strength copper for ich on clown loaches often with great success. And I’ve often used antibiotics and anti-parasitic meds full strength successfully on loaches and scaleless fish over my 50 years of keeping fish. Two owners of fish stores (Vicki of Freshwater Aquariums and Cory of Aquarium Co-op) have both treated thousands of scaleless fish with full-strength meds with no problems.

Aulonocara saulosi
Aulonocara Saulosi

Medications and Beneficial (nitrifying) Bacteria

Methylene blue will attack beneficial bacteria. If one has to use it one needs to either move the filter off the tank or do a fish-in cycling after using it. Most other ich medications do not affect beneficial bacteria.

“Effects of Parasiticides on Nitrification”, Collins,  et. al., 1975.

Treatment of four recirculating systems with a single dose of methylene blue at 5mg/liter resulted in complete cessation of nitrification for 16 days as evidenced by a rapid rise in ammonia and stable nitrate concentrations. The therapeutic use of methylene blue in recirculating systems is contraindicated. Treatment of the systems with therapeutic levels of formalin, malachite green, formalin and malachite green in combination, copper sulfate, potassium permanganate, and sodium chloride had no effect on nitrification.

Antibiotics are surprisingly benign with regard to beneficial bacteria. All the research available says that antibiotics have to be in very high concentrations to affect nitrifying bacteria in a well-established filter with a lot of “brown gunk” in it. If the filter is NOT well established antibiotics will kill the beneficial bacteria and one will need to do a fish-in cycling. This is not a big worry as it is easily done by just limiting the amount of food fed.

I do not encourage turning off the filter when treating diseases. Turning off the filter will often simply make the disease worse.

Since antibiotics are decidedly more effective when put in the food there should be little problem with nitrifying bacteria if one is treating with antibiotics. The concentrations of antibiotics in the water column will be very low.

Aulonocara jacobfreibergi Eureka Jake Peacock
Aulonocara jacobfreibergi Eureka Jake Peacock

Prevention is the Best Treatment

The best “treatment” for fish diseases is to prevent them from occurring in the first place. And the best preventative for fish diseases is to have very clean aquarium water with a dirty filter. And “clean aquarium water” does not mean an aquarium where the water is changed 75% three times a week. It means an aquarium where the water is heavily biofiltered, bacteria-free and crystal clear.

We go into this very important topic in more depth in this article:

12.9. Avoiding Fish Diseases

The Shotgun Approach

Sometimes an aquarium has a series of slow deaths that just can’t be explained. There are no symptoms of disease, the water is crystal clean and bacteria-free, and the aeration is great. The best course of action at that point is something called the “shotgun” approach. The hobbyist just treats for the three most likely treatable pathogens. This approach is covered in the following link:

12.10. The Shotgun Approach

Startpage Aquariumscience

Source: – David Bogert

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