I decided to experiment and used aquarium type gravel in the 2nd last chamber of the filter box that was provided with the tank. It worked well and the pH was always maintained above 7 and that saved alot of work for me. Of course coral or other forms of shells may also work but having heard so much about the dangers of habouring parasites in these media, gravel may be an alternative. It's easy to clean and cheap and for the small filter box provided with the tank, you don't need alot of gravel, maybe just about 10kg will do. Put a layer of sponge at the bottom to support the gravel and put the gravel into a plastic net so that you could remove them easily for cleaning anytime.
Lets analyse the issue carefully.
I guess all filter media would certainly trap fish waste for its intended purposes, be it ceramic rings, coral. The question here wld be maintenance. Since we bag the coral in nettings, cleaning shld not be too much a problem for a filter box. The purpose of using coral is to maintain a balanced PH in the water. I agree that green mat make cleaning easier and it is light weight.