First, are you feeding frozen foods? You have to rinse it. Thaw the cube out in a cup of RO water, then dump the thawed food with the water in a fishnet. Pour another cup of RO water over the food in the fish net and then feed the food to the fish by inverting the net in the water.
Only use RO water, not tap or well water, for rinsing foods, water changes or top offs.
Your canister may hold nitrates but I never heard (that does not mean it can’t be so) of phosphates being held by canister media.
I do my water changes and if wheI test, I see any phosphates, and I do mean any…I use a product called Posbuster pro or PhosphateX. I run Purigen in my canisters, it is expensive but reusable and IMO works better than anything on the market. I only needed the phosphate removers maybe 2Xs in a year because the purigen and water changes took care of it like it should.
My phosphates come from the frozen foods I use, and things got much better after Meowzer told me to rinse it before feeding the fish.
I hope this helps.
