too many plants to medicate?

[Soeman]

I've got a 30g moderately well planted with additional duckweed and a water lettuce floating on the surface. I've also been at the front lines of a long lasting battle against pervasive brown algae (diatoms).

The other morning I noticed that my fishes had been afflicted with ick, and I quickly treated the tank turning the water blue (as should be expected). When I came back from work aroudn 6 hrs later, the water was clear. Is it normal for ick medication (API liquid super ick cure) to lose its hue after a few hours, or are the plants and algae absorbing it all? If the medication is being taken up by the vegetation, is it safe to add more?

Obviously, the ideal situation would be to move my fishes to a treatment tank, but at the moment it is not an option. It should also be noted that I do _not_ have a carbon filter.