Ummm I use PhosGuard & on the bottle it says after using recommened dosage for your tank.
Leave product in place for 4 days, then test phosphate or silicate concentrations. If the concentration of the component you are trying to decrease has not dropped to around 0.02mg/L, then replace the PhosGuard, otherwise leave in place untill levels begin to climb again. As long as concentrations remain under control, the product is not exhausted.
Thats what it says on my bottle of Phosgaurd, and like I said above I tested for phosphates found that they were arouns 1.0ppm, did an immidete water change, and added phosgaurd. Next day phosphate test showed .50- .75ppm which is getting lower for one day, did another water change, on my third day my phosphate levels were below .50 and did not do a water change. By weeks end into the 8th day my phosphates were as low as I could get them with this product and water changes and now my Red slime problem as gone away.
The product that I'm using worked really well in my tank, and I'm happy with its work on getting my phosphates into control in as little time as possible.