Any form of degrading organic matter has the potential to release nutrients into the water. It really doesn't matter where it lands. If it stays in the tank, then it will degrade. The only reason you might care where it lands is 1) if you want it to be easily removed and 2) if it lands on a *healthy* sand bed or in any heavily critter populated area of the tank, it will be "processed" more quickly. (Seriously, if you ever have the chance, look at your sand under a microscope... it's like a huge, busy trash collection facility/recycling center down there-- millions of little organisms busy doing their thing). The "seed shrimp" are my personal favorites. They are just so cute and fun to watch.
But anyway... no, sorry, I've never heard of Prodibio, is it a phosphate removing media? If it is, and if it's similar to Seachem's or any of the ones we have here, then it shouldn't have caused a problem (unless it was used for way too long and started leaching out phosphates it had removed).
Anyway, I think using a *good* media to remove phosphates is a good idea. I use SeaChem's phosphate remover because it works well in my Phosban reactor (media reactors are awesome btw!). Just don't get TOO comfortable... you see, there are two types of phosphates- organic and inorganic. These commercial phosphate removers will remove mostly inorganic phosphates. This can give you a false sense of security because the common aquarium-use phosphate test kits only test for inorganic phosphates. Thus, you might think you've removed all the phosphates from your water when you might still have considerable organic phosphates.
In any case, congrats on getting that horrible stuff under control!
