Hi all,
These are a bit like the UV lights that they sell to pond keepers to kill off their green water. They will work, but they are really "snake oil", because they are just treating the symptom, not the cause.
Like Lorne says, the API one is a flocculant, probably a long chain polymer (polyacrylamide, should feel "greasy")? and causes the bacteria/clay colloids etc to clump together into "flocs", which are then filtered out by the filter. You can get the same effect with thick filter floss or a diatom filter.
If your water is cloudy, there will be a reason, and it might be better to find out what the reason is first.
cheers Darrel