Hi,
Kuhli loaches!!!!!
They love to dig into substrates and can be very nocturnal.
Mmmm.... Maybe Paul is on the right track.
When are you dosing your trace elements as sometimes a reaction can occur between the P in your macros and the Chelator of your micros, if its an EDTA Chelator that is. Normally happens in hardish water but I've had it in soft water when relatively large ammounts are used. Clears of its own accord within 24hours.
Are you running a UV steriliser on your tank by any chance?
Regards, Chris.