I think you need to do a bit of work with what you have now before moving to a reef. RO/DI is a good start, but, you do need to lower your nitrates. Large water changes (30-40%) every 2 days for several cycles, with truly matching pH, T, Sal, are OK, especially as you have no inverts, and your fish (BTW, what do you have?) will be fine. Mix and aerate well, use the same thermometer for tank and SW (even if the thermometer is off by 1 or 2 degrees, at least the 2 water sources will still match), and use a calibrated
refractometer. When changing you r water, siphon up as much slime as you can and blow it off the rocks with a turkey baster while siphoning. Add an extra power head to flow, especially into areas that are relatively stagnant, although flow alone will not cure or prevent cyano/algae. Nutrient control is the key.
You're correct that your algae outbreak is from both excess nitrates and probably phosphates in your system. The elevated phosphates are only inferred as you have no test results for them. Get a test kit. Even a Salifert can guide you (I know, yes, Hanna is more accurate, just trying to be cost effective for now). However, even with RO/DI makeup water, you're still adding phosphates every time you feed your fish. Run a phosphate removal product. Phosban is really popular, and a phosban reactor is not expensive. Do NOT run phosban (or other GFO media) in your Fluval, The flow rate will be too high, and the pellets will be ground to dust and released into the tank (not a healthy thing). Actually, as your waste management is an issue, toss the Fluval. Too must detritus gets built up in it and slowly breaks down, adding more nitrogenous waste back to the tank. Unless you're willing to clean it out once a week and use it for carbon, you really should lose it. As you plan on getting set up with a sump, you really don't need it.
Issues of how much LR, lighting, can all wait until you have matters in hand. A skimmer would be helpful right now in getting the tank under control, but I don't know what to tell you on that, as buying a HOB skimmer, when you're adding a sump in the near future, seems wasteful to me. You could get a high quality HOB skimmer and then move it onto the sump, but a skimmer designed for a sump is usually more cost effective and can offer better performance. I'll let others chime in on that one.
Finally, you have a larger problem at hand. Where are you getting your drinking water from?? Nitrates of 20 ppm exceeds EPA standards by double. Never mind purifying water for the fish, you need to address your water source for the humans in your home first. I know you care for your tank and its inhabitants, as do I, but right now you have undrinkable water and that should be your first priority for your own health. You'll get your tank in shape, but invest in some bottled water for yourself, and see about setting up your RO/DI to produce drinking water for yourself as well (no, you can't just drink the water right from the final DI stage).
Where do you live and get your water from?
Henry
