Oblong..

Here's what I can tell you (may be of limited value, since I run coldwater macros):
Lighting: what you describe is more than adequate for macroalgae that I've encountered. No actinic required here (assuming algae only tank, no corals etc..)

Livestock: I've added green crabs and hermit crabs to make things a little interesting and to have a bit of bio-load in the tank.

Substrate: most macroalgae don't care about the substrate since they take their nutrients directly from the water column. That said some Caulerpa have runners, and some other vascular plants like Zostera like a fine substrate since they have roots, proper. Depending on the algae you intend to keep, it will come attached to a chunk of rock or something via the holdfast. Some types will regenerate a holdfast if fixed in place (though I can't say I've successfully achieved that - yet!)

Water movement: Of paramount importance, in my experience ESPECIALLY with the coldwater types. Without vigorous water movement, fine deposits seem to accumulate on the thalli/fronds and they don't seem to do very well after that. I've had grape caulerpa with less water movement, and they fared considerably better.

Although filtration isn't necessary with the lighter bio-loads, skimming seems quite important. I pull a nice cup of peasoup out of my skimmer every couple of days, especially at this time of year when some of my specimens die-off until spring.

Dosing: CO2 definitely improves things, as does iron (Fe) and iodine (I). I occassionally add Kent Trace Elements, although I'm not sure why (I think I do it more for myself.. . Some calcareous (sp?) types obviously require supplements too. I have one alga that grows on a rock (Corallina officinalis) that is a reddish-purple with white calcified tips. When the calcium in my water is low, it is evident in the alga's tips.

I hope this is of some value to you, as it can be hard to find information on macroalgae sometimes.

Do you have any particular algae in mind for your setup?

Cheers,

RAS