Flashing cories

[Pidge]

Hello,
I've got 8 cory pymaeous and 4 pandas in a 40litre. Although they're really active, feed well and not shy, a couple of weeks ago they started flashing/scratching against the sand or plants every so often (no constantly).
My water parameters are nh3=0, no2=0, no3<5-10, ph=7.5. I do 25% water changes twice a week. I've tried treating with a esha200/exit combination to rule out white spot/flukes etc and then I tried sterazin to no effect.
Any thoughts most welcome, thanks.

[Pidge]

Sorry, I should have added that other occupants are 6, 1 month old platy fry. The substrate is silver sand. Furnishing consists of driftwood, various anubius, some pebbles. The tank is kept at 24-25C, and filtered with a fluval 1 plus and an air driven sponge filter. I don't know the KH/GH

[Aquamick]

This COULD be due to the small particles of silver sand getting among the Armour plates of the corys and irritating them.Just guessing, but I saw this once when I had a flashing goldfish.under a magnifying glass, i could see the particles of sand.Just Guessing though!

[Caol_ila]

I've tried treating with a esha200/exit combination to rule out white spot/flukes etc

Hi!

Thas was a rather bad move in my eyes...as the Esha/exit products contain copper which is really bad for the cats...also treating w/o seing ich isnt the right choice.

[worton[pl]]

hiya.
I noticed that fish sometimes flash/scratch after fed with flake food :]