I would stop using the cartridge that came with it and replace with a sponge. Might have to strip off the cloth filter material and place it in the filter with the sponge. This will help it to seed if it already started. API ammo lock will work fine. You could use ammo chips to absorb the ammonia this way your test kit will not show false positives by using prime or ammo lock. Don't bother replacing the carbon. It actually removes some trace minerals and should only be used to remove medication after treatment. The original purpose was to remove the fish smell from the water and because in the old days of aquarium keeping it was believed old water was better and water changes were frowned upon. So they used carbon to remove what they can instead of doing water changes.