Use about 5ml (1 tsp) per gallon. A little goes a long way if you are mixing it with other stuff.
I think the reason slime seems to be more of a problem now for hydro users than before if because of the market leaning towards organic nutrients and additives. You used to hear people going to a bit of trouble to ensure they had all organic nutes for their grow, because so much was synthetic. Now days it's sometimes difficult to make sure there are no organics in your line up. Kind of a flip flop. Anyway that is just a guess. As for chloramine, generally it isn't much of a problem, but what you're saying is possible if the local plant is using high doses. You can request a water quality report from them.
I have also heard a few conspiracy theories. The govt/cartel/CIA released a pathogen that targets cannabis. Aside form the usual problems I have with conspiracy theories, it seems rather odd that simple soil microbes can overcome a genetically engineered pathogen, not to mention the slime does not actually infect plants. I am always dismayed when people convey information based on the sensationalism rather than the truth value, and hydro shop owners seem especially prone to do that.