Hawkian is absolutely correct. Water clarifiers work by chemicals that bind together minute invisible particles (the stuff that makes the water cloudy rather than clear) so that they can be better trapped in the filter pad. All very good, except... these chemicals do the same on the fish's gills. At the very least this is highly stressful, at worst it can outright kill some fish. Think of it as someone preventing you from breathing without enormous effort and discomfort, like holding a thick cloth over your face. Very bad.
Many years ago before I knew this I used one of these things; I now understand why the fish immediately swam and huddled at the opposite end of the tank. They were attempting to escape this suffocation. I did an immediate pwc which settled things down again.
It is a good principle to never put any
chemicals in an aquarium containing fish unless it is absolutely essential (such as water conditioner). Chemicals that allege to clear water, kill snails, kill algae, remove sludge and organics, whatever, do not belong in an aquarium.