Phosguard? It's a very short term solution to get phosphates down quickly. A better solution would be GFO, Brightwell's X-Port PO4 cubes, Brightwell's PhosphatR, for short term, Brightwell's Phosphate-E, the list goes on for phosphate remover. I'm using a Fauna Marin product called Ultra Phos 0.04 Phosphate remover and it's working well, states that it won't let PO4 drop below 0.04, though I have yet to test that.
If you use Phosguard you should add it and test it over 3-4 days. With the other products (minus Phosphate-E which you have to test daily), you don't necessarily need to test that much, but it's not a bad idea to test a lot if your parameters are out of whack...