The phosguard and all Al based products would not leeech Al into the water. Unless you tank is very very very sick. It is basic chemistry. Although I am too busy to get my analytical book out of the basement I can offer a basic explanation.
Think of your Ca reactors. Inside the rxn chamber the water is being pushed down to very low pH's. Al is the same. At low pH the Al will begin to leech into the water. We are talking like 4-5.0 Noone I know of could ever get a reef tank down to that nor would they want to.
To tell you a story.... I worked in the ADK mtns on some lake restorations. These people were pumping the raw sewage into the lake. O.k. when there are 5 cabins on a lake. But late 70's early 80's = economic boom and people have got $ to buy land and cottages all the suddent that turned to like 50 cottages. So... still pumping sewage in the lake. Wham phosphorus spike and bam! Enormous cyanno blooms on these peoples little ex-pristine piece of very expensive property. Enter, ADK park ageny, and EPA. Now they literally dumped Al based phosphorus removers into the lakes. Phos lowered and now treating waste from cottages, algae gone everyones happy. fast forward...... enter the elimination of all buffering capacity by way of acid precipitation and natural leeching of tannic acid from the evergreens. pH of the lakes drops suddenly and sharply. Low and behold now fish are starting to show edema (found the book) reduced gill atpase activity that "reduces active transport of Na, Cl into plasma from water". By the way the magic number (or negative log rather) is pH = 5 before the Al will start to leech into the water. DEC's answer = lime the lakes. Basically just a large dose of kalkwasser. In the end things still aren't right. So moral of the story don't return the water from your Ca reactor anywhere Near where you have the phosphorus remover placed. Otherwise there is really NO WAY that it would leech Al into the water. JMHO
keep on swimming,
Ben
p.s. I gotta go get some PO4 remover also. I can't really comment on which sucks more phosphorus.