I use a sponge filter only on my daughters 20 gal, a dozen guppies, a few shrimp, snails, and 2 juvie bolivian rams.
sponge filters provide a little mechanical filtration but will provide all the chemical filtration you need unless you are really overstocked.
are you sure the sponge was cycled?
20L My Other Tank: OEBT Shrimp
40B Inlaws Tank: 2 goldfish
75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry