Regarding plants and oxygen levels, there is no correlation there, reverse phase photosynthesis takes up O2 at night, so its a wash.
Thanks Jay. I thought about that before I posted, but I'm not sure I'd call it a wash. Here's why - First, over a 24 hour period the overall plants have a net intake of carbon. So if that's from CO2, I presume (don't know) that that implies a net output of O2.
But more important, the excess of O2 caused by plant respiration is indeed in the daytime... which happens to be when most fish can make better use of it. That as opposed the night when they are generally dormant, in low O2 demand states.
Either way, I would not call it a wash, since the fish can spend a large part of their environment in a high O2 environment. Many things that are therapeutic to animals are not constant events, but benefit is incurred through regular repeated occurrences. That could be the case here.
But with all that said, upon reflection... I seriously doubt algae output of O2, even for the active portion of a fish's day, could make that much of a difference. If only because that would have to be one seriously algae choked tank to make much of a difference in O2 levels. And as far as the Discus correlation goes, I realized that you don't see hole in the head disease in discus tanks that are even "sparsely" planted. Again, not much of a difference in O2 for a few measly plants. So I doubt O2 is the key.
HITH in discus and oscars doesn't have the same progression as HLLE in marine fish. In discus, it can routinely be cured by feeding the fish metronidazole. The presumption is that Hexamita protozoans in the gut become prolific and take up too many nutrients that the discus needs. Using metro on marine fish does NOT cure them - thus a different cause is at work.
But if that's so... all the O2 discussion is moot. But I did not know Hole in the Head was hex. Thought it was different in fact. And an say that I've had discus - in planted tanks - that had what appeared to have hex, but never HLLE. More points to ponder.