If you suspect your tank is infected with flukes; you dose once, wait 5-7 days, do at least a 20-25% WC, and then dose again. The reason for the 2 doses is Prazi kills worms, but not the eggs they leave behind. The 5-7 day window usually ensures you get the "second generation" before they can lay eggs of their own. This is far from 100% certain though. I've known people who have had to treat 4-5x to completely eradicate certain strains of flukes (myself included).
I would follow the directions on the bottle; some people get away with overdosing, but it can cause problems.