What test are you using for ammonium? I think regular nitrifying bacteria should work (microbacter xlm or dr tims one and only or turbo start 900 will be the fastest). I know zeolite removes ammonia (so maybe it removes ammonium) but it not a good long term solution.
Also, I would repost this in the chemistry section. I think Randy may see it there and he tends to have a lot of good info on anything chemistry related.