So, should I shoot for 2ppm of Phosphate using one of the liquid types
that my LFS has?
I'm probably the wrong one to ask. I never have to supplement phosphate,
because my tap water has tons of phosphate in it. (Some hobbyists would
consider that a nightmare, but I consider myself lucky.)
Seachem has a new phosphate supplement for planted tanks that's supposed to be
very good. It's probably expensive, though. A lot of people recommend using
Fleet Enemas, from the drugstore. Really!
As for dosage...most people go for "barely detectible" rather than trying for a
specific number. A lot of people are scared of adding phosphate, because they
think it causes algae. I have to say, though, that it hasn't been my
experience. Despite the 40 ppm of orthophosphate in my tap water, I've never
had serious algae problems, in planted or unplanted tanks.
Still, keep in mind that any changes should be gradual, so you don't shock the
fish. Add a little at a time.