It really depends upon the type of fish.
Rheophilic fish need very heavy flow, with an ATF or cichlids like Tomocichla or Theraps, it would be hard to ever produce too much flow.
For discus or certain other oxbow, or swamp fish, it may be too much.
In some cenotes containing Jack dempseys, uropthalmus and live bearers where I have snorkeled, I have had to hold on to mangrove roots to keep from being swept toward the sea.