I use a Seio multicontroller to reduce to flow rate on my (2) Koralia 4s. It reduces the output 50% for a user set timeframe (15s minimum - 30 min maximum).
My blenny once got stuck on the outside of it but he's nosy so he gets himself in a lot of trouble. But I wouldnt think that any healthy fish could get caught on it, fish are strong enough swimmers that they would be able to swim against the draw of the koralia. My sailfin loves it and a good place to purchase is championlighting . com and the last time I went to New world aquarium about 2 weeks ago they had some also but NW is pricey.