If your tank breaks and collapses, then you have too much flow. In all seriousness, it probably depends on what you are keeping. If your tank is 100% sps then you probably can't create a (realistic) situation in a tank that would be too much flow. You certainly shouldn't aim powerheads directly at sps. Obviously, if your tank is lps and/or softies, then yes, youcould have too much flow.