Algae Problems

[Palster]

First off let me start by saying I am somewhat new to saltwater fish keeping so I this is kind of a newbie question. I am currently experiencing an algae bloom in my tank. I have two types on nuisance algae. One is what I believe to be green hair algae. The other is what I believe to be purple slime algae. Ammonia is 0, nitrites are 0, PH is 8.3 and the test kit is showing low levels of Nitrate (near 0) however I tend not to believe this as this test kit is old and I suspect inaccurate. The tank is 90 gallons with a sump and approx. 90 lbs of live rock. All the fish in the tank are healthy. The tank has been running for about 3 months but all of the live rock and live sand used during the setup were from an established system (most of it was from my 50 gallon which had been running for quite some time with some additional fully cured live rock added as well). I had been using a Prism protein skimmer ,which did not seem to working well, up until a few days ago when I switched to a Berlin skimmer. I have also been using tap water as I do not have a R/O unit for purifying the water. From what I have read I suspect the problem is a nitrate and phosphate issue caused by both the inefficient skimmer I had been running and the tap water. I think I have solved the one problem of the skimmer as the Berlin skimmer seams to be working well as of today. My question is if I was to go to the purified water store and purchase bottled water in 5 gallon containers (I have lots of 5 gallon jugs which I can fill for 2 bucks apiece) would this help with the water quality (the purified water store uses a reverse osmosis system) and also how much water should I be changing with each water change (I was thinking of doing 20 gallons). This would be a band aid solution until I purchase a reverse osmosis system. I live in a rural area and we have a small community water system where the water is pumped out of a creek so I have no idea what the water quality is like.