Hello fellow fish keepers, I'm writing this article in hopes that there is someone out there in the community that can explain to me the difference in ammonia vs. ammonium. Firstly, my aquarium is a 55 gal. freshwater. (Non chiclid) I am aware that compared to ammonia, ammonium is not harmful to the aquariums inhabitants. Secondly, I have measured my ammonia levels just a very short while ago and have noticed that the levels are .50ppm. (point 50 that is!) a while ago it was steady at .25ppm. The tank has been running for aprox. 4 weeks now and yes, I have live plants in it as well as aprox. 14 fish which all seem to be doing well. I may have rushed in and put too many fish in the tank at once. At the time, my conception was to purchase two (2) black mollies and leave them be to start the acclimation of the water. It was about two weeks later when I introduced another (glo-light tetras (4) then a mystery snail and some more plants. (JFYI, I did do a saltwater cleanse on the leafs and stocks of the plants) About a week later I added a pearl Gourami and because he didn't like others in the tank, (constantly chased them) I removed him and replaced it with two smaller gourami - one powder blue (beautiful!!) and a honey. I also picked up a bristle nose too. Tiny one at that! My filter is a HOB MarineLand Emperor 400 and I have modified its medium by using only an aftermarket 100% polyester material. The filter has two chambers which allow for a vast array of media options one could use. I decided to build a divider made of 1/4" plexiglass - drilled holes throughout to allow for water circulation and used the fluval balls (or whatever they're called!) inside both left and right side chambers. I kept the bio wheels in place as they will provide with a great air/water mix medium. Secondly; When my water started to increase its ammonia level (according to API test kit) I added a couple capfuls of Seachem Prime to help combat the ammonia levels. As I mentioned earlier, the level - according to my API test kit, measured .50ppm. I should also mention that i do not use charcoal media of any kind but have installed a large "400" bag of ammonia rid from aquaflo inside one side of my filter. I read somewhere that American pharmaceuticals (API) test kits can NOT decipher between ammonia and ammonium. Grrr!! Anyone out there know how to distinguish the difference - a fool proof method would be greatly appreciated! Thanks to all who respond.