I disagree as well. Good flake food is very nutritious, and need not have any more phosphate than anything else. Some does have extra phosphate, especially if it has anything with bones in it (like fish meal), but many do not.
All foods contain substantial phosphate, but unless they have added phosphate (like some fresh, canned and frozen seafoods for people do), I am not certain that any particular type of food has more phosphate relative to other nutrients like protein than do other types.
Phosphate is an integral part of many tissues, not just a preservative. It is a part of proteins, phospholipids, DNA, RNA, etc. There is no getting around that. So anything that contains fish, seaweed, clams, etc is going to contain phosphorous. And the higher the protein, the higher the phosphorous is going to be, typically.
Fresh, canned and frozen seafood from the grocery store does often have phosphate added to retain moisture in the food. So be careful of that.
You might want to avoid anything with bones in it as they are very high in phosphorus. So fish meal as an ingredient might be something to avoid.
Ron Shimek analyzes some foods here;
http://web.archive.org/web/20010720071031/http://www.animalnetwork.com/fish/data/foods.asp
From it I calculated the phosphorus to protein ratio, and get:
Formula Two...........................176
Formula One.............................69
Frozen Brine Shrimp................232
Frozen Plankton/Krill...............182
Golden Pearls..........................273
Lancefish.................................244
Nori..........................................221
Plankton Gold Flakes...............169
Prime Reef.................................69
Saltwater Staple......................500
Silversides...............................952
Tahitian Blend..........................318