Originally Posted by
m0nk
It's possible that you're introducing it through the salt you use. Although most salts say "No Phosphates" or "Phosphate Free" some of them to in fact have them and in high amounts. One that's apparently notorious for this is Instant Ocean. I used it until a friend, who's a Marine Biologist, told me that when he was in college they did wide ranges of tests on freshly mixed salt water using various salt mixes and IO always came out with high phosphates. Just a thought, but perhaps you should test your saltwater right after you mix it. Hope that helps!
I had the same problem when my LFS ran out of Oceanic and I bought I.O. instead . My tank was a disaster and found it coming from the salt . I threw my premixed water and the entire bucket of salt out . Ordered some Oceanic and did a bunch of water changes along with some phosphate removers . It was a bad way to find this out and costly .