Why are my goldfish dying?

[Chris Topher]

System is fully cycled and I have about 40 small goldfish.  The ammonia/nitrite/nitrate are all where they should be.  The PH is about 7.6.  The tank is 150gal and is about 100 gallons full w/ two 50gal grow beds.   Lots of aeration with 4 stones. The only extra thing I add extra besides fish flakes is some maxicrop at about 2 oz per day.  I have 2-6 dead per day and have to keep adding more goldfish although a handful are doing fine and seem to have grown some.  I should also add that although I first filled the system from the city water, that was 3 months ago or so and I top off a few times per week with 5-10 gal of rain barrel or well water.

My only thought of how they could die would be from the pump but I have yet to see one stuck to the side of the pump and it never stops. If they were dying from the pump I don't know that they would always be floating. They also don't look like they suffered trauma. I want to figure it out before I invest in Talapia.

[Kenny]

I did notice about 5 days ago that one tilapia was basically dead, but when I netted him, he opened and closed its mouth.  I then put him in a holding basket and had a source of tank water purging the basket... it seemed to work so I thought it was an airation problem and added the air stone and pump.  Just wondering why I'm still having a die off?  Since I added the air, I experienced 2 more that died.

Thanks

[matthew ferrell]

Like some mention before fish die after stress.  I call it the 3 day rule.  If something bad happens to the fish, 3 days later you will start to see the death.  Depending on whats causing that will determine how long your in the woods.  Low DO can cause this sort of event.