Zebrafish colony start-up problem

[Beth Ann Sipple]

Hi fellow zfish researchers,

My name is Beth Sipple and I am currently starting up a zfish 
colony at Saint Vincent College for my student Betsy who is 
investigating various aspects of early heart development.  

However, we hit a snag today with the colony and I was hoping that 
I might be able to get some hints from you all on what to do.  

The tanks were thoroughly cleaned & filled according to the regular 
zfish guidelines.  The 10gal tanks ran with the correct size filter for 
3 weeks prior to introducing the fish.  All 40 fish were placed into 
one of the tanks and they were to be subdivided in one week. pH 
and temp have been stable.  My student has been in charge of 
tank maintenance and she updates me daily.  However, after 5 
days the water became cloudy, whitish cloudy not algae cloudy but 
the fish seemed fine.  I planned on contacting PetSmart on 
monday to see what I should do to help clear the water but there 
did not appear to be an urgency since the fish appeared active & 
healthy.  Then a call from Betsy today (Sunday) informed me of a 
few deaths.  I had her move the fish to the back up tank (on a 
separate water supply) and to put a water sample and the dead fish 
in the frig.  The fish and water are going to Pet Smart tomorrow 
afternoon (we are not using any standard wild type strains like *AB 
for her work).  They have a 14 day warrantee (that sounds odd 
doesn't it).  Anyway, I am not sure what happened but I am 
thinking that 1) overfeeding led to a build up of by-products or 2) the 
filter was not able to handle clearing the wastes from 40 fish in the 
one 10gal tank.  

It has been over 4 years since I have handled fish and I don't even 
have a tank at home.  Any hints or suggestions on what might be 
wrong and how to fix it would be extremely helpful!!!  

Thank you for your help in advance!

Beth Sipple