So I was away for a couple of days and when I came back my apisto had died. All other inhabitants in this 12 gallon tank were fine. So what I did was that I took him out, I did a water change, normal maintenance - cleaning the gravel/substrate and so on. I also used my test kits for NO2 and NO3 - both showed 0ppm. Now approximately two days after I found this white fungi looking stuff on the substrate/on some driftwood.. look at the picture attached. Does anyone know what this is? How should I get rid of it (will my shrimp and snails take care of it?)? Is it dangerous for the fish?