I'm currently fishless cycling a 5.5 gallon tank with ammonia. The cycling is going just fine, but now I have debris in a tank that does not have any fish and has all new substrate, decor, and filters. I put a 2 lb container of TOPFIN Aquatic Decor gravel in the tank. I think it is broken shells with the edges rounded and smoothed. I think some of the debris may be from these shells, but I rinsed everything before putting it in the tank. I also have two small Jardin sponge filters. Here is a link to the filters I'm using. http://tinyurl.com/nkbggdp.
I have two of them because I hope to use this tank as a quarantine/hospital tank for new fish to put in the 10 gallon. But now there are little black dots all over the shell gravel. Could it be from these new filters? Perhaps I didn't rinse them thoroughly enough before putting them in the tank? Anybody have any idea what this could be and would it be harmful for any fish I put in the tank? i