i would say okay seeming you dont have co2 yet but when you get co2 and after a few days and the plants adjust and start making o2 properly then lower it atad not the flow as such but the surface movement
, surface movement is perfect for what happened to you today i think
Don't think i've ever seen a tank with too much flow. Its just a case of checking your stocking (eg, dont put weak fish like fighters in high flow tanks etc).
Seen a 205 running full whack on an Elite 35L tank wouldnt have said that was too high a flow as the stocking was right for a high flow tank!
Could be a lot of things... *any* of the water parameters out can make them gasp, stress can do it... gill parasites etc can do it... if they are dying from something entirely unrelated... that can stress them out and make them gasp...
I shouldnt have been offline, didnt realise you had so many problems