Brown spots on white gravel / substrate
New aquarium owner here of a 40 gallon cube type tank. The tank has been running for about 11 days or so. We did the turbostart and also have added stresscoat / quickstart, etc and checked the water levels before we started slowly adding fish. We now have 5 zebra danios, 3 neon yellow danios, 3 otocinclus, and 3 cory catish. Yesterday, we started noticing some brown spots on the white gravel. These came about a day or two after we added the otocinclus and cory catfish -- we were told to feed them algae wafers once or twice a week until our tank starts building enough algae for them to eat, and I guess the catfish will also eat any other remaining fish flakes that fall all the way down to the bottom? I think we may need to get a pellet type food for the catfish that sinks? Anyways, getting off topic -- our question is what are these brown spots -- is it a brown algae? Is this normal? There were a few algae wafers down in these general spots at first but the fish eventually consumed them slowly, and pretty much all of the fish in the tank were into them. Thanks in advance!