I have never used it personally but the sales blub reads:
"In every aquarium there are potential disease causing organisms, including species of parasites, fungus and bacteria. Liquisil General Tonic keeps these disease organisms at an acceptable level and thus prevents disease outbreaks."
Kind of a scare mongering sales pitch IMO, those sorts of organisms wouldnt build up to levels where they affect the fish if the water is changed regularly. The second to using something like this, in the case of Ich say, if a product like this is used, if it fails and an ich outbreak occurs (adding new fish or some thing starts it off) My concern would be the parasite has built up an immunity because of this product being added therefore making an ich medication less effective.