What is in the fx6? If you aren't running some sort of high surface area media i.e matrix, lava rock, or the like, then you don't have enough surface area. Additionally, using straight di water is detrimental to your beneficial bacteria colony unless you are doing water changes every couple of days. Your Ammonia is going up because you don't have enough of a colony. You don't have enough of a colony because they have no where to live. If I were you, I would fill up the sponge chambers you have, and any other space with decent flow, with lava rock.