I'm currently using two AquaClear 110's (each 900 gph) situated side-by-side on one end of a 125g tank. One is placed slightly higher than the other, so that if you look through the opposite end of the tank you see two powerheads looking at you like a "%" sign. This gives me strong flow right to the other end of the tank, with a bit of 'disruption' (can't think of a better term to describe it) where they collide, which seems to eliminate any dead spots. My skimmer is placed at the same end as the power heads, just above the lower powerhead.

You'll probably have to research the water conditions that your particular algae prefer when deciding where/how to place your algae. In my situation, I had to make sure that the Codium were in waterflow, but not too strong a flow. On the other hand the Ascophyllum did best where the water was moving along nicely.

Chaetomorpha seems to like any kind of movement but can get tangled in just about everything, and is best left in the refugium, or restricted in it's movement somehow (I've used mesh onion bags, but they're unsightly).

I suspect that the smaller tropical algae out there will be less demanding as far as circulation goes, but I can't say for sure. At least with the tropical species, I don't think you'll have to worry about seasonal die-off periods that seem to be common with coldwater species. Hopefully someone else will be able to verify this.

RAS