I happen to like phosguard better than gfo, I never had any luck with the gfo, it always clogged and took too long in my opinion. lol
I would put the reccomended amount of phosguard in a filter bag and then add to one of the trays in your canister. Are you running a canister fulltime? If so, they do need to be cleaned quite frequently like mentioned above. I ran a fluval 305 on my 29g and then my 55g and always had 0 nitrates. It can be done, just a bit more work.
I would either look into a portable ro/di unit or see what your lfs charges. In the long run, it does pay to buy an actual unit, it will save you alot of money. Plus its nice to be able to make water in an emergency like on a sunday night after the lfs is closed, sometimes that can be a matter of life and death for your tank.
here is a sample of what I found on ebay
0ppm Portable 100GPD Reverse Osmosis Ro Di Filtration | eBay
I used this very unit for over a year until i had enough money to buy a big bulk reef supply setup. I also make a crap ton of water, lol, so it was necessary for me to get a larger unit while you can probably get by with the one I linked.