Q. I have a few questions on fish medications. When a fish medication requires a series of treatments over days or weeks, most books and drug manufacturers recommend a 50- to 75-percent water change between treatments. Doesn’t this dilute the drug and render it less effective? Shouldn’t the medication be allowed to build up over time? It is also my understanding that fish medications are generally non-selective. If this is so, how can these drugs kill harmful bacteria but not hurt the nitrifying bacteria in biological filters?
A. You raise two important questions here regarding the effectiveness and risks of using fish medications. The first issue involves the chemical stability of drugs dissolved in aquarium or pond water. As it turns out, most of the available medications are not particularly stable in fish-rearing waters. Dyes, such as malachite green and potassium permangenate, are deactived very quickly if there are high concentrations of organic material present. They may only remain active for a few hours. Other drugs, such as kanamycin sulfate and nifurpirinol, may remain active for several days. Factors such as pH, temperature, light intensity and organic content of the water can radically alter the stability of fish medications.
The re-dosing schedule suggested by a manufacturer is an attempt to compensate for the natural breakdown of the product. Each dose is intended to provide the manufacturer’s recommended concentration on the assumption that any prior treatment has been rendered inactive. This results in a stable concentration of drug in the water. Thus, partial water changes prior to each new treatment will not dilute the intended dose.
The reason a partial water change is highly recommended is because some of the breakdown products of fish medications can be harmful to the fish. Changing the water dilutes these chemicals before the new dose is added. More often than not, the frequent water changes recommended as part of a drug treatment regimen do more to help the fish than the drugs themselves. Indeed, many aquarists have given credit to ineffective fish medications when the real curative was the water changes!
In general, drug treatments in backyard ponds are far less effective than drug treatments in aquariums. This is because the various factors already mentioned — temperature, light intensity, organic content, etc. — are less easily controlled. For example, ponds will have a much higher organic load than the average aquarium. In the same vein, treatments in a specially set up quarantine aquariums or hospital pond will tend to be more effective than in rearing aquariums and backyard ponds.
Your second question gets to the heart of a basic concern of all aquarists and pondkeepers: protecting biological filters from the harmful effects of fish medications. As it turns out, most fish medications are far more specific than manufacturers tend to claim. In any case, controlled laboratory tests have shown that very few fish medications actually harm the biological filter when used as recommended by the manufacturer. In some cases, this is because the nitrifying bacteria are just not sensitive to the drug. In other cases, it is because the recommended dose is too low to affect these or any other bacteria. The only commonly available fish medications that have been shown to have highly lethal effects on biological filters when used at recognized therapeutic concentrations are methylene blue and erythromycin.