[Mass cory die off overnight]

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.

[Mass cory die off overnight]

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.

[Hello! I just discovered this forum and I am excited to be here!]

I own a 5.5 gallon with one male betta, a 10 gallon with two female bettas (and four corys up until last Friday, I'll get to that later), and a small pond with three large goldfish. 🙂