bio filters (live rock, bioballs, whatever) turn ammonia into nitrite and then nitrite into nitrate. if you have nitrites, but no ammonia, wait a while and it will all convert into nitrate. it's then much safer for fish. inverts are pickier when it comes to nitrates, though, so you may have to pay attention to the next (and last) step in the cycle, which is the conversion of nitrate to nitrogen gas, which bubbles off and goes away.
54g all-glass bowfront corner planted FW
Red tiger lilly, Rotala Indica, Sagittaria Subulata, Micro Sword, Cardinal Plant
Fluorite Black sand and gravel