08-25-2008, 11:21 PM
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Well, there are a number of different culprits that are believed to be responsible for hole-in-the-head disease. However, water quality is a major issue. By biweekly, do you mean twice every week or once every two weeks? What do your water parameters usually look like. It's hard to tell from the picture, but how big exactly is this oscar? I would say that due to your tank being overstocked, your water quality may not be the best.
The oscar will eventually need about 50 gallons of water on his own and should be in a tank of at least 75 gallons because the fish gets to 16" in length. The parrots will get to about 9" each. If the pleco is a common pleco, it can hit 18" and needs a 75g tank. What species of gourami do you have? Also, what kind of fish is your knife fish? I'd say that you're going to need a much larger tank for all of these fish, possibly very large if you've got something like a clown knife fish and possibly completely huge if you've got giant gouramis.
Also, your oscar's stomach looks a little concave to me in that picture, which is a sign that he's not getting enough to eat. How much do you feed him? Malnourishment has been linked to hole-in-the-head so that could be a factor as well.