why are my fish dying?

Question
I forund 3 fish dead last night 1 was the biggest the other 2 where my algae eaters. I had replaced 1/2 the water yesterday, treated the water and everything seemed fine. Now all my fish are swimming at the top of the tank, even my snail are at the rim if the water level. I checked my ph, amonna and other levels they are normal or in the safe zone. But after I changed the water yesterday I found 10 to 15 baby fish. Which was a suprise because I had thought I had been buying only males. could this make difference and why are all the fish dying and now only at the top of the tank. Help Please I have only been doing this for a 6 months and I run a daycare so these fish are the kids pets and they are cring over the lose of the 3 I need to find out what's wrong and get it fixed quick . thank you!

Answer
Hi Keri,
A lack of oxygen will cause the fish to hang at the surface. Seeing as you run a daycare are the children putting anything into the tank that could be harmful to your fish? When you say your water readings are fine what exactly did they read? Ammonia and nitrite should be at 0 and nitrate under 20ppm.

Regards,
Chloe.