Check out this website. It lists all kinds of potential areas where phosphates can enter the tank from food to plastic bins to live rock.
HPO4Sources
Here's another idea for you to try. Try to isolate the problem. First off, you've ruled out the water source, but what about the buckets for mixing the salt. After you have established the water even salt water is okay with no phospates, turn to the tank.
put live rock in one container, activated carbon in another, calcium reactor media in another, plastic bins by themselves, anything you add to the tank, test it to see the phosphate level. If it's the food, change brands, if it's the plastic bins change them.
Just ideas, food for thought.