Do you know what the flow ont he return pump is?
I think you would do well to add some more / bigger circulation pumps regardless.
How are you managing water changes? Do you have an RODI
unit, are you buying from a LFS
or grocery store, etc? What you don't want to do is use tapwater. It has all kinds of stuff in it that while generally harmless for us becaome fertilizer for algae.
It sounds like you have some kind of test kit with at least ammonia, nitrite, and nitrate. If you don't have already, pH, alkalinity, calcium and magnesium are also receommended for keeping corals.
As far as the light you referenced, I can't tell what type or how many bulbs that fixture has. If it is a T5HO fixture (which is generally the cheapest initial outlay of cash), I'd recommend something with at least four rows of bulbs. (Probably an 8-bulb fixture on that size tank. I think on 6-foot tanks, eacdh bulb is typically half the length of the tank.) More is better. If you go metal halide, you probably want at least three bulbs of at least 250 Watts each. These are next up in terms of initial outlay. If you can afford the initial outlay, LEDs are the way to go. Althoguh the initial cost seems sky high, you don't have to replace bulbs and they don't heat the water like halides will.
The brittle star is probably fine. You don't often see them as hitchikers, but the green sepent star is the only one of that complex to look out for.