Here is something out of left field. I work at a chemical company where we have some large water softening, reverse osmosis and de-ionizing equipment. This equipment is serviced by a local water treatment company that sells home and industrial equipment.
During one of my many conversations with the owner and employees I learned that they do a lot of water treatment in hospitals with kidney dialysis machines. Apparently these machines use lots of activated carbon which is frequently changed out. According to them this carbon still has 95% of its life left but is still replaced because of the strict requirements of the hospital. For something that important I don't blame them. This carbon is then thrown away by the bagful.
Activated carbon is the same thing you find in your aquarium filter cartridges which can absorb ammonia. I plan on trying a RAS next year after I move and have talked to our water guy about getting some of the carbon. I already have an inside track on this cause the water guy wants to keep me happy as I am his contact for service calls.
Of course there are dialysis machines all over the country and companies that service them. So just maybe if you contact the right place and talk real nice you might find a supply of free or cheap activated carbon that you can use in a RAS. Now how much is needed or how long it will last I have no idea. Maybe next year I will have experience with it but for now I just wanted to throw it out there.
I doubt the Hospital Carbon could be given away due to the extreme bio-hazard.
Also IIRC,carbon does not absorb ammonia. Zeolite does. Ammonia needs bacteria to be broken down into less toxic nitrates which other bacteria break down into nitrites---I may have the nitrate/nitrite thing reversed.
