Fish Tank Problem Solved?

I have written a few times about our persistent algae problem in our fish tank. Officially, it's "green water," which is green algae in the water but not growing on the sides of the tank or the stuff in the tank. We have spent a few months trying to get rid of it, using all sorts of ideas from Internet research, and nothing works. On the up side, the fish don't mind it and three are surviving - one has been with us since Thanksgiving. Ben finally asked about it at Jack's Aquarium Tuesday night. They said it sounded like phosphates in the water (not sure why our $40 water test kit does not test for phosphates), and so Ben brought them in a sample of water to test at the store. They concluded that our water does contain too much phosphate, so they sold us a special filter and said within two weeks the green water should be clear. After that, we only have to do water changes once a month (Ben had been doing weekly), and use the special filter for the first week of the month.

So Tuesday evening Ben put in the new filter. And sadly, there has been no change that we can notice. We keep telling ourselves it's only been a few days, and it's supposed to take two weeks. The fish are fine, but we would like to see them, and the water is so green that it's hard to find the fish unless they are right up against the glass.