I'm kind of surprised that nobody has forwarded the idea of mechanical failure of the fish's body wall opposite of the air bladder. The one-sided bulge makes me think of that as a possibility. To tell you the truth, this is kind of a newish idea for me, I heard of a couple of cases and then heard of some more here recently on RC. A picture would really help....especially since NexDog described the bulge as being in the stomach - which would rule out the airbladder all together!
TealCobra and Percula9,
You're both kind of correct, but both kind of wrong. It is all a problem of dosage. Nobody really knows how much water marine fish drink, therefore it isn't possible to create a proper oral dose by just treating the water and hoping the fish will drink enough of it. Baths work for external infections only, and remember that fish don't drink much at all during hyposalinity. The variables are tremendous; temperature, salinity, stability of the antibiotic in solution, etc.
Adding the antibiotics to the food is NOT to concentrate the medication (which actually could be toxic), it is to get the CORRECT dosage. All antibiotics have a dosage range usually expressed in mg of antibiotic per kg of fish. You need to hit that range (with some degree of flexability) or you risk either not killing the bacteria or killing the fish. Injectables are also dosed as mg per kg.
Antibiotics in food can work in some cases because you can calculate an estimated food amount eaten by a fish as 1 or 2% of its body weight per day, and then feed that amount, with enough antibiotics added to reach the required mg per kg dose.
All else is just a shotgun approach, and rarely worth the effort (or risk). Some bacterial diseases are self-limiting, so the fish gets sick, you do your thing and it gets better - but it was going to recover anyway(grin).
Bottom line: if you do not use the weight of the fish at some point in your antibiotic calculations, you simply are not dosing it properly.
JHemdal
