
Get a bigger skimmer!
If you're doing 50% water changes with 0 phospahte water yet your tank still has high phosphates then there's another source. I assume you're not feeding the snails, where did you get your rock from? It sounds like your rock or sand (or something) is leaching phosphates into your water.
Isn't this method a little indirect? Skimmer cannot skim phosphate by itself unless they are already inside some algae or something more tangible and then got skimmed out to the cup.
