GFO (granular ferric oxide) and phosban/phosguard (aluminum oxide) help to reduce the nitrates in your tank. The simplest but not most effective way is to run it in a media bag placed in a high flow area of the tank. They will not harm any of your inhabitants but be sure to rinse them well so you don't get the silty leftovers floating around. Depeding on which one you use, you'll use it from 4 days to 6 months but that also depends on the levels of phosphate in your tank. More phosphate, the less time you'll get with the chemicals as they'll saturate faster.
The foxface in your tank kind of depends on how often you feed it. The more you feed it, the faster it will grow. They can be coral munchers though so if you're just using it to get rid of the algae, then get it out sooner than later if you have corals. The size of when you get it out just depends on you and how comfortable you feel it is in that size of a tank.