i would advice u to be patience in this wonderful hobby ... if ur tank is fully cycle , there will not be a fluactuation of nitrite or even ammonia...
in the meanwhile , stop all stocking , get a nitrite tester , only when it reach 0ppm, then u start stocking up slowly ...
sponge feed on iodine ?? from what i know , they feed on phyto and iodine is a element that will help to 'colour ' them up ....
tank cycling is different from tank to tank , so make sure ur ammonia and nitrite is zero first ...