My husbands oscars

Question
My husband had two oscars in a 55 gallon tank who constantly had hole in
the head and ick.  The water was constantly off balance.  He had a diatom
filter and another filter.  The fish laid eggs and now we have a 40 gallon tank
with about 15 four inch oscars.  The tank and filter were bought at a garage
sale.  The tank is constantly filthy and has a terrible odor.  It smells so bad my
son and I don't even go in the game room anymore.  He says that many
oscars in a small tank is fine, because they will not grow large in a small
tank.  He just feeds them 25 little pellets each day so they wont get big.  The  
large filter bag is filthy.  I tried rinsing it out twice a day, but it didn't help the
tank.  One day he did a partial water change and the water was black.  I was
wandering how often I could do a water change in the tank?  Every day?  
Every other day?  My goal it to be able to spend time in the game room.  I
know nothing about keeping fish and he seems to think he knows everything.  
Please help!

Answer
Hi Karen,
  Yikes, that is a dangerous one for me to answer -- I don't want to be the focus of a confrontation but here are the facts.

  The idea that keeping many fish in a small tank keeps them small is correct but it is not for a good reason.  Basically, they become stunted. This is very unhealthy for them and if done for too long, they will never become normal functioning adults.  I strongly recommend against doing this.  

  For a tank with oscars, you need to change 25% of the water once a week, EVERY week to keep them healthy or else you are going to see hole in the head down the line and there won't be anything you can do about it then.

  In the short term, if I encountered that tank, I would change 50% of the water today, then 50% again tommorrow and 50% again the third day.  Then go to the regular 25% once a week, every week deal.  A fish tank should not smell bad.  DO NOT CHANGE more than 50% at once -- that could easily kill the fish.

  Good luck :)

-- Ron
  rcoleman@cichlidresearch.com
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