Could you be overfeeding the fish by quite a bit? Fish food can rot and breakdown to nitrates and phosphates. If your tap water is low in both, and you have a well planted tank, with the plants growing well, there shouldn't much buildup of any of the fertilizer components. If there is, it has to come from something, and overfeeding is just one possibility. If you have any common snails in the tank, their population should be booming if you are overfeeding that much.