08-24-2009, 02:51 PM
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Not good adding medication to a tank unless you know exactly what the problem is. Corys are sensitive to any chemicals and medication, and could be worse. One thing is fairly certain to me, whatever it is was probably the result of not maintaining the tank. Corys especially are sensitive to water quality and parameters. Neglecting weekly partial water changes allows nitrates to build up (and several corydoras experts have commented that nitrates appear to affect corys) and that is food for the algae as you've seen. Not to mention that the fish are swimming around in their own waste (urine and solid broken down). The first order of business is to rectify the water quality.
However, some caution is needed or this too can turn out even worse. What is the pH of your (his) tap water, and what is the pH of the tank water now? The latter has probably lowered. If it is below 7.0 it is acidic, which is not bad in itself but it means that the ammonia in the tank is actually ammonium. Ammonium is basically non-toxic to fish (unlike ammonia). If the tap water pH is above 7.0 (basic, or alkaline), adding new tap water during a partial water change will cause the pH in the tank to rise and if it rises above 7.0 the ammonium immediately converts back into ammonia. This can result in dead fish either because the shock is too great or the ammonia is too much, depending.
If the pH of the tank water is reasonably close to the tap water, and both are either above or below neutral (7.0) a partial water change of 50% of the tank will do no harm; use a good water conditioner. Scrape the algae off the glass before syphoning out the existing water, and vacuum the substrate at the same time. In the event the pH is far apart and crosses the neutral line, do very small partial water changes over time, and use a water conditioner that detoxifies ammonia (Prime does, there are a couple of others, some do not). Ammonia is very toxic, it burns the fish gills. Which sort of brings us back to the cory, this could be its problem, or any number of other things, but the first thing is to rectify the tank's messed-up biology.