Dying cories!

[jgarden2000 at juno_com]

I have some albino cories in my plant tank (see? this is still on-topic!)
that have are dying and I am totally stumped why. 

The fish will look perfectly fine, except that when it sits still on the
bottom, it starts to float up, and it has to work to stay down. Sometimes
they will tip over and lie on their sides. After a couple days, they're
dead. 

I lost a batch several months ago to this, about one fish dying every
week, and now one of the six I have now died yesterday. I'd love to
figure out what's killing them before I lose the other five.

The tank is a 40-gallon "long", heavily planted. No other fish, but there
are some ghost and Amano shrimp, Malaysian trumpet and pond snails, etc.
The substrate is normal gravel with laterite mixed in; the laterite has
worked its way to the top and floats up in clouds whenever the cories
root for food (I try to siphon it off when this happens). I also noticed
some bubbles come up from the gravel when the fish dug around, but I
don't notice any H2S smell (just an icky "fishy" smell...). I was adding
yeast CO2 and liquid Kent "Freshwater Plant K + Fe" solution, but I
stopped both when the last fish died.

Anybody have any ideas? I'm stumped!

William

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