Oscar is lethargic and not eating

Question
I thought of one followup question to ask.  My father is a biologist, and has identified several (millions?) little creatures called ostracods in my tank.  Apparently they are harmless.  They look like little lice or fleas, but they don't seem to attach to my oscar.  They seem to attach to the glass, instead.  Water changes take care of this rather well, but they always come back within a few days.  Other than trying to completely start over (with no holding tank while the new tank gets setup as far as biological filters), do you have any ideas about how to get rid of these creatures, or whether they pose positive or negative effects on the tank?  (They may raise/lower levels of nitrates/nitrites?)  I don't know if you've heard of them, but I thought maybe you could help.  Thanks.

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Followup To
Question -
My oscar is not himself.  Normally he patrols the tank.  Moreover, when we feed him the occasional group of feeder fish (a treat, not his staple), he rips into them.  Now he barely moves, except the occasional spasm against the bottom of the tank or any of the fixtures in the tank.  He doesn't seem to be eating.  I've noticed no external spots, clouds, or anything of the sort - he looks like a slightly light-colored version of a very healthy oscar.  I've noticed he opens his mouth very wide at times now.  It seems the only normal thing he does is pick up and spit out pebbles.  I've tried an antibiotic, but it doesn't seem to have had any effect.  Do you have any suggestions?  He is about two years old, and he is about 8-10 inches long.  It seems like he's dying, now.
Answer -
Hi Michael,
  I'm really sorry but there isn't much I can say.  It's possible that he is having digestive problems.  I have heard, but never tried it, people feeding their oscars thawed frozen peas as a way of helping them get through rough spots. You might try that.

  And as always, some good water changes are one positive thing you can do. Oscars are very sensitive to water conditions and you need to be changing 20% of the water EVERY week to keep him healthy but I assume that you have been doing that.

  Occasionally larger cichlids go through periods of depression for no obvious reason.  I have the same trouble with several of my large, old cichlids.  The best thing you can do is to keep offering him some food, but be sure that uneaten food does not pile up in the tank, and keep up the water changes.  

-- Ron
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>


Answer
Hi Michael,
 Sorry for the delay in responding; I thought I sent an answer but evidentally it got lost in the system.  

 The presence of large numbers of small invertebrates generally indicate that there is a lot of excess nutrients in the water.  Water changes will help this.  It is a common problem with oscars because oscars eat so much and produce so much waste, as well as being "messy" eaters in many cases.  

These small creatures will not generally cause any harm and will not affect the chemical balance of the tank.  Think of them more as warning signs that the nutrient level is too high.

-- Ron
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>