For sand sifing gobies to do there job the sand has to pass through their gills other wise there is no sense in getting one if you have corse sand. I like the little guys and want one, because I think it would starve in my tank because it can't sift the sand for it food. It could take in fod directly through it mouth but main corse of food should come from the sand. I read a some thing in a book once about this specific fish need specific foods some time to thrive in a aquarium.
As for the flow try changing the angle the koralia 3's are pointing that may help. Red slime algae and diatoms don't thrive in high flow tanks, you should just try and play with the angle the koralias are pointing. suck up as much as you can before changing the angles first.
Bill