For no reason 3 of my 4 Peppered Cory Cats and my 4 Otto's died today. I've had the corys since November and the Ottos since the beginning on March.They have been living in a cycled tank Ammonia 0, Nitrites 0, Nitrates 20ppm with my platies, neons and glofish. I went to do a water change and when I moved some of my breeder grass, three of them just floated up to the top of the tank. Just the other day they were swimming around like normal and then yesterday they were sleeping on top of each other, and today, dead. All of the other fish are looking good and are very active, so I'm not sure what's going on. Does anyone have any ideas to what could have killed my corys?

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Never under estimate the power of an empty fish tank

what was your KH and PH? If you have very little KH, it will allow high ph swings and that could kill them. Also, do you have enough water flow and oxygen in your tank? Did they all come from the same spot when you saw them dead? Maybe there was some bad anerobic spots in there and they found it...