If the powerheads are setup correctly there should be no sand storms or fish flying around. I find my 425 kinda weak in my cube actually. But its a shorter tank than a 29 regular. I keep mine aimed at each others flow and towards the front or back glass. You want a totally chaotic flow going on in there.
I've used purple up since its so milky to see where the flow actually goes and where it doesnt.
You dont want any dead spots on the sand or rocks. And you also wanna make sure your corals are stable so you wouldnt' point a powerhead directly at the corals. My clowns LOVE to play in the flow, they do it all day in my 125g with 1400gph powerheads. back and forth all day long. LOL
It's like surfing I guess for them.