Too much flow exists if corals are being blasted and negatively affected. Also if fish have a very hard time swimming not able to keep. Flow is very beneficial to a reef tank and the more the better until inhabitants start to become affected. Softies and lps prefer lower flow but can also do well in higher flow areas. Sps prefer high to very high flow. Flow keeps things from settling and keeps organics suspended so the skimmer can remove them. Always want to eliminate dead spots.