Getting New Fish

Hi everyone, not been on here in a while, been fairly uneventful here.
Sadly though, four of the Pygmy cories I bought back in September have died. They are a really lovely fish, but I wonder now if my water conditions are quite right for them. Or maybe it's because I needed to rethink my feeding regimen.
Previously I fed flake, flake, frozen bloodworm, starve day, crushed pea, flake, flake, frozen daphnia, starve day etc.
But I didn't figure that the Pygmy cories may not be eating on flake or pea day, so I got some sinking pellets and now feed them on flake days.
The snails are loving them!
Also a couple of days ago the pump on my shrimp tank gave up, so my shrimp are in the aquanano too.
Ie 10 (ish) ember tetra, two CPD (found another dessicated jumper behind the sideboard recently) 2 Pygmy cory, 4 amano shrimp.
I also wonder if there could have been something wrong with the cories.  On three occasions I saw a fish acting crazy, going up and down the tank, flipping into its back, playing dead on the sand, then swimming away normally. The next day after this I found a dead fish. I've assumed it was the same one. The other missing cory has just disappeared without trace.
I've googled this behaviour without much of a result.
What do you think... Is there something obvious to you guys that I'm doing wrong? Water changes are 20%weekly, ammonia and nitrite always zero.