· snails are your friend
Brass is an alloy of copper and zinc, the first of which is deadly to shrimp. What I don't know is at what rate brass can corrode and release free copper into the water. I believe that the blue to greenish patina it develops is due to copper oxidation, but unsure at what level it could leach into your water. I would replace the fitting with PVC or plastic if possible. If you did want to seal over, that would need to be done out of water and given ample curing time while dry. Sorry I don't have an exact answer, but I sure wouldn't chance it.