Rumply has been doing a wonderful job of guiding you so I will just leave it at that.
About asking for more opinions about RO I will chime in. Using RO guarantees you have a starting point with your tank. It means you know EXACTLY what you put into the tank with your hands. If you end up with an algae outbreak you know it's not from the water supply. This hobby works the best when we know and control our outcomes.
A sudden outbreak of algae need attention.
First, always check your source water for unannounced contamination. Well with RO you know your source water and unless your membranes broke you no longer need to worry about source water.
Second, did something die? Visually check the tank for missing fish. No missing fish or large inverts? OK then on to the third check.
Three, are you overfeeding?
Fourth is your skimmer and other such business in good order?
The less steps you have the easier to maintain, get it? So with clean source water you eliminate a HUGE amount of the guess work. And in general a few good water changes with clean water can help clear a tank up quickly.
Good RO units are not cheap. Cheap RO units are not good. You get what you pay for in life. If you have a local shop that sells it for $1 for 5g then that's great. It just might be easier and cheaper to buy it. That's what I did for 2 years and was extremely happy doing so. With rising gas prices and longer work weeks it proved cheaper to buy my own unit. I bought a 300gpd $700 unit. I can tell you that it now takes 40 minutes to fill a 5g jug instead of the 2 minutes it used to take at the LFS. In 40 mins I could fill 50g of water and shop. I certainly miss the convenience and speed of already having it at the shop. But I just bought a large container like at the shops and can now fill a jug in a few minutes time but my RO unit works like crazy to refill the main container.
If shopping for a RO unit buy a RO/DI unit. Produces laboratory grade water. Should have 0TDS, total dissolved solids. An RO unit can run a discharge of 5-30TDS before needing to be replaced. The deionization resins take it to 0 and hold it there. When shopping for a unit make sure it uses DOW FILTECH membranes. The 75gpd membrane is the most efficient. It produces the cleanest water. The 100gpd membranes (such as mine) produce more water but it has a higher TDS. A good DOW filter runs about $50. You should also get an HMS dual inline TDS meter. They tell you about the status of your filters. Otherwise it's just a guess as to whether they are working or not. If you buy an RO only set up place one probe on the intake side of the RO and one on the "clean" side discharge. Most say to install one on the home supply side, why do it like I say? Because the membrane is the most important part of the system. Who cares what you put into it? You care about what comes out. So sure it might be fun to to place the first TDS meter at the supply and see it drop from around 400TDS to 4TDS at the jug side but then you are also measuring the sediment filters as well as the RO membrane. So when the 4TDS at the "clean" side begins to creep up do you replace the filters or the membrane? When you first start the unit up with the TDS meter installed as I say, write the input TDS on your sediment filter with a marker. When it begins to creep up it is time to replace the sediment filters. This saves and extends the life of the membrane to years instead of months.
If you buy a RO/DI unit place the meters at the intake and output of the DI cartridges. Any increase going into the DI and you need to replace the sediment filters and possibly the membrane. Any increase at the output and your DI resin is spent and needs replacing. This way you monitor the output from your membrane and your resin. Chances are your resin will need to be replaced at a ratio of 2:1 of your membranes.
Please check out Buckeye Field Supply for the best online information about RO/DI units. They also sell an economical RO/DI unit for $125 and that's a great deal including the DI resin and a Filmtech membrane. http://www.buckeyefieldsupply.com/sh...ry=167&Sub=166
It's only about $20 more then the RO only systems.
read the "more info" PDF as the units do not come supplied with connection and drain hardware. As they put it, each system is unique. I recommend the option to remove the cold water hose from your kitchen sink and add the quick tap and extra valve. Get the drain saddle as well as it makes a convenient waste disposal port.
Other options. If you have a laundry room the Y adapter is an easy choice and you can put the discharge hose right into the same open drain your washer goes into.
If you have a freshwater tank do as I do, I drain out 75g of water from my 125g fresh tank and run the "waste" line to the tank. The "waste" water is actually cleaner then my tapwater and has been treated for chloramine by my carbon sediment filters. I actually kill 2 birds with one stone. Better for the environment and easier to justify in my wallet.
These guys are hands down the best I've ever met. I bought my unit from somebody else used and I'll tell you they got onto an online forum and completely walked me through the entire system until I found a torn membrane. Super nice and friendly people.