guppies DEAD

Question
hi
sadly that guppie died, we changed the water and found brown sludge in the filter, could it be food because we change food about a couple of weeks ago could it be that. now this morning another guppie has died and it was fine yesterday. we checked the pH levels and the nitrate was up and the temp was a bit low. what could the nitrate be it is up to 230 please help. thank you  from Jamie  
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The text above is a follow-up to ...

-----Question-----
hi i have some guppies and one of them has stop swimming and is sitting on the bottom of the tank. i don't think it is eating either. what could this be? please help
-----Answer-----
Hi Jamie,
 Has it been attacked by other fish?   

 Are you doing regular weekly water changes?  You should be changing 20% of the water once a week, every week.

 Also, be sure that the temperature in the tank is around 80oF.

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


Answer
Hi Jamie,
 There shouldn't be sludge in the filter -- that sounds like there is way too much food going uneaten in the tank, or else you aren't cleaning the filter very often.

  Which kind of filter do you have and how often do you clean it?

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