Did you change the filter media, do a full gravel vac, and/or clean all decorations? If you do 2 or all of these things it will kill a lot of the nitrifying bacteria and can cause a mini cycle. If you clean nearly everything it's probably even possible for a whole new cycle. Oh, did you use any meds? That could also kill the bacteria off.
Current fish (includes all 3 running tanks): Polypterids, severums (of which I raised from eggs), gouramis, plecos (gold spot ones & a gurupa), loaches (zebras, yoyos, & a kubotai), macculocchi spotted silver dollars, an African butterfly fish, & Ctenopomas (a leopard & an ansorgii)