If there is no mechanism in the Fluval to reduce flow (I own two of them and haven't a clue), you can place a sponge on the filter intake and try to reduce flow that way. I have done that for my shellies in a 10 gal
with an AquaClear 150. If the flow is too great, the sand gets blown everywhere. Using the switch on the filter and the sponge, I have cut back the flow enough that the shellies are living in calm waters once again!