RO/DI unit would be the next thing on my list. As long as your skimmer is a big enough size, and from what I understand, your light is a good light, the RO/DI should come now. I wouldn't even worry about your flow right now...that can come later. Get the water quality up first. Get all the test kits you need too. Eventually, you may want to do something with kalk or calcium reactors, but again, your 2 part can do that for you as long as you're faithful in measuring your levels.