My thoughts: I doubt that there is any ammonia and if be surprised if there are nitrites. You should have enough rock in there to support the bio filter assuming there is enough water movement to give the bacteria oxygen. I'd take a sample to the local lfs assuming he uses salifert, and get it tested. No need to buy an ammonia kit if it tests 0. If you do buy a kit, get salifert. Does the black algae look like it's caked and come off in a broken sheet? Slimy? Could it be a very dark green or blue? Could be blue green algae. I'd like a better characterization of the algae.