you will find it hard to keep your nudis alive with enough sponge to feed them
a cray of legal size will need at least a 600 litre setup
and a chiller will be essential
viewtopic.php?f=5&t=23675
have you changed water supply area or water in the tank move
i think carbonates hold a fair amount of the hydronium ions in check. IF there are less carbonates in new water they can be used up by plants and the hydronium is released causing a crash