I agree with the RO system as being a priority. I would also look at a RO system with a De-Ioniser filter as well (RO/DI). The de-ioniser removes what the RO membrane can not.
A UV sterilizer is simply a bright ultra violet light that the water passes through. This intense light will kill any bacteria and algae spores that pass through it. They are useful at eliminating freefloating algae spores in the water and will also prevent most diseases from taking over a tank. I got mine from Lowes and is an 18watt unit that is sold for outdoor ponds. It works very well on my 72 gal tank. IMO they are not a necessity but they really make me feel better about the cleanliness of my water. I do not have an algae problem at all and I have never had a disease outbreak.
Another thing you can look at is an ozone unit. I have a 25mg/hr Sander ozone unit that I run into my venturi protein skimmer. I simply took the air line coming out of my skimmer and hooked it up the the ozone unit. Ozone works at making a protein skimmer more efficient. The ozone rapidly breaks apart much of the dissolved sludge that is dissolved in the water. It also kills bacteria. As the ozone breaks apart the contaminates, it is changed from O3 to O2, simple oxygen gas. The oxygen gas then diffuses in the water which makes for very high oxygen levels in the water. This is both good for your fish and bad for any algae that tries to grow. If you use ozone, your water will become completely crystal clear. You will need to buy an Oxidation/Reduction Potential (ORP) meter if you decide to use ozone. This will give you a measurement of how much ozone is actually in your water. You want to have just enough ozone to form oxygen gas, not too much so that you actually have excess ozone dissolved in the water. If you actually smell ozone coming from your tank, then you are using too much.
I have used ozone for 3 years and I absolutely love it. I keep mangroves in the refugium without any problem and have many invertebrates as well. I have not seen any consequences to using it and my water is totally transparent.
But anyway, I would get a RO/DI unit before I would get into anything else. It's best to start with clean, pure water rather than trying to add extra devices to clean up dirty water.
