sick Mickey Mouse fish

Question
My male Mickey Mouse fish has been acting funny for a few days.  He stays at the bottom of the tank almost all the time and isn't eating. He looks like he has gotten smaller. When he swims, it looks like he is struggling.  I have tested the water, and the other fish seem to be perfectly normal.  I am a new aquarium owner.  We set our tank up in March.  It is 20 gallons, and we have three guppies, three Mickey mouse fish (one of those is a baby born in our tank), and an algae eater.  I don't know enough about fish to know if there is anything I can do, and I am having a hard time finding answers online.

Thanks

Answer
Hi Kim,
  That's really hard to say what is wrong. It could easily be that he is getting old.  Mickey mouse platies do not live that long.  

  Are you doing regular partial water changes?  You should be changing 25% of the water in the tank once a week, every week, to keep your fish happy and healthy.

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