By listening to the advice of your LFS, you have placed yourself back near the beginning of a fish-in cycle. Since they take about a month to complete, if you change the sponge again, your fish will never enjoy a fully cycled tank. The cloudiness will clear on its own in a few days but the ammonia will not. Keep a close eye on ammonia and nitrites and use water changes to clear up an excess of either one. Next time you think it is time to change the sponge, try rinsing it out in old tank water, the water you throw away with a water change, and return the sponge to the spot you took it from. You will find that you do not get a fresh bacterial bloom and do not get any ammonia returning to the tank. Instead, you will have preserved the bacterial colony that you have worked so hard to establish.