Oscar loses appetite

Question
Hello,
I have a large oscar about 11 inches in a 29 gallon tank. I know you will frown upon a tank that small but a very good fish book said one oscar can be maintained in a 29 gallon IF he is the only inhabitant which that is my case as well.

My Oscar also gets at least 30% water changes about 3-times a week. My problem is one day he'll be very hungry and then the next time he won't touch anything but most of the time he is the latter.
Why would he do this? I feed or at least try to feed Hikari Oscar pellets, Krill, Shrimp pellets, and a couple of other foods. In the past he ate them with gusto but now maybe since he is an  he requires less food, would this be a reasonable explanation for him? He did have a strange ammonia spike for some strange reason possibly for leftover food he wouldn't touch but now that is fixed and he seems fine other than not eating.

Thank you for your time and help!
Your suggestions are greatly appreciated!

Susan~

Ps: The tank is well established after a year of being setup. My oscar has spent his whole life in there.
thanks again!

Answer
Hi Susan,
 You are right, I am frowning (about the size of the oscar relative to the tank).  However, you are doing a lot of water changes, which is very good but surely must be getting tiring?  

 What concerns me is that with that much fish in that size of a tank, you are only making it sustainable by doing so much water changing, i.e., you are standing on the edge of a cliff. If for some reason you can't do that many changes in a week, your tank can go over the cliff very quickly, taking your poor oscar with it.    

 I can't say for sure, but it wouldn't surprise me if his appetite is actually tracking the levels of ammonia in the tank and that he is in essence feeling good some days and not feeling good other days.  This is a rather roller-coaster way for him to live and it may not be sustainable in the long-term.  Remember, oscars can live
into their teens.

-- Ron
  rcoleman@cichlidresearch.com
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