I Bought What I was Told, Why Are My Fish Still Sick?
Successful resolution of an aquarium health problem involves one of the following:  either blind luck; or the fish would have recovered whether you did (or in spite of what you did) anything or not; or else a correct series of events involving diagnostic and treatment choices.  Since we can’t do much about the first two, I’m going to talk about the last one.
The steps involved in successful treatment of problems are:
1) correct identification of the problem
2) correct choice of therapy
3) therapy (such as drugs) contain sufficient active ingredient
4) therapy is actually getting to the pathogen, in sufficient quantity to kill it without being of harm to the patient
5) problem is treated for adequate length of time
6) conditions are optimized for the patient (this is not absolutely necessary but will definitely increase your chances of success)
As you can see, this makes the whole thing a little more complicated than it seems on the surface.  I’m going to discuss each of these points in more detail.
1) Correct identification of the problem.  This is WAY harder than you might imagine.  Many things look like many other things, and especially when one is going by a description given by another person, it’s difficutl to be accurate.  Access to a microscope and a book with good pictures helps; also, common things are common (that’s why most people can identify ich, for instance).  Not all red streaky fins mean septicemia, though, and not all cases of septicemia are caused by the same bacteria.  So if a disease isn’t responding as expected, the first step is to rethink the diagnosis.
2) Correct choice of therapy.  This step contains several implications – that it has any effective therapy at all, or that this particular strain is susceptible to the same things the usual strains are.  Bacterial infections, particularly in freshwater aquarium fish, are becoming increasingly resistant to the average antibiotics used (and this is partly the fault of being bombarded with antibiotics on a random basis).  Aeromonas and Pseudomonas, for instance, are common pathogens, and are notorious for developing resistance to drugs.  You might read in a book that most bacterial problems in fish are caused by Aeromonas, and that Drug  So and so kills it, but that may no longer be the case. 
3) The therapy you have now acquired is actually any good.  If you’ll notice, treatments (especially antibiotics) are labeled “for ornamental fish use only”.   There’s a good reason for this – the purity and strength of the drug may not be being monitored particularly closely, and indeed it’s possible that it is outdated or contaiminated. 
4) Proper delivery mode and adequate strength.  By and large, treating an internal infection (such as septicemia) with an external bath (putting meds in the water) is useless.  The fish’s skin is designed to keep foreign substances out, and the therapy simply isn’t getting to where the problem is.  Either buying a prepared medicated food or making your own (if the fish will eat) is more likely to be of help.  An exception to this is certainly problems like flukes, or columnaris while it is still external; baths may well help here.  As for adequate strength – commercial preparations are made with the idea of avoiding problems associated with overdose, so they are deliberately made to dose on the low side.  This may well be inadequate for treating many problems.  And realistically, little is known about adequate dosing in fish – absorption, drug breakdown, drug toxicity is probably different from species to species, and very little is known in non food type fishes.  Things tend to be extrapolated from trout and catfish  and salmon, and may not be truly valid.
And some things work just fine, but can be toxic easily – formalin (formaldehyde solution) is a good example.  It’s a pretty effective killer of many bad things, but unless used very carefully, can be pretty hard on the fish (the environment as well).  So you really have to know what you’re doing to use it.
5) Problem treated for adequate length of time.  Often, the signs go away, but some of the bad guys are still present; if treatment is stopped too soon, they can come back with a vengeance (since it tends to be the sturdier one who are killed last).
6) And finally, the fish with the immune system that is functioning at its best will have the best chance of getting it over the problem.  Optimal living conditions will help the immune system. 
So, as you can see, a failure in one of these steps will lead to treatment failure.  Develop a systematic way of looking at problems and formulating a treatment plan, and you will have the most likely chance for success.

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