Mass cory die off overnight

[sketchaire]

I don't know what I did, but here goes:

For about 6 months I have had two female betta and four albino corys in a 10 gallon tank with no issues.  A month ago I switched from a sponge filter to a hang-on-back AquaClear; I have very hard water (180 GH) and I wanted to be able to add a water pillow to soften it.  Then two weeks later I changed the substrate from gravel to sand (prepackaged aquarium sand from PetCo).  Everyone looked healthy happy and strong, eating and swimming well.  Then on 12/24 I woke up to find all four corys dead with no sign of injury (the two betta are perfectly fine). 

What did I do to kill them?  Was it softening the water too quickly (went from 180 GH to 40 GH in a week)?

Ammonia: 0

Nitrates & Nitrite: 0

pH: 6.0

Temp:  78

Thanks for your help.  I loved my little corys and I want to be able to keep more but I'm afraid of killing them again.

[HH Morant]

Sorry about your Cories.

Changing the filter and substrate within a month results in the loss the huge majority of the beneficial bacteria, which can cause an ammonia/nitrite spike. But your water parameters say that did not happen. And at a 6.0 pH, it would take a lot of ammonia to be toxic to fish.

The only thing I can think of is the possibility that something toxic was introduced with the new substrate, which might mean more to the cories than to the bettas, since the cories are usually in contact with the substrate.

If you keep fish in an aquarium you are going to lose fish, and the reason cannot always be determined. I think that when you get knocked down, you just have to get up and try again.

[sketchaire]

Thank you.  My immediate thought was an ammonia spike and I was shocked when all the parameters came back fine.  It must have had something to do with the new sand and maybe the filter.  I wonder if anyone else has had a problem with toxins in sand.