I used to mix RO and tap water half-and-half. I didn't really measure; I just filled an empty gallon jug about halfway with tap water and then filled the rest with RO water. I didn't do it to get rid of algae, but to lower my very high pH a little.
Your tap water nitrates are 0. Did you get a phosphate reading at the lfs? Since you say you had lots of algae, then what Newfound says is my hunch too - your water either contains too many phosphates or some other mineral like iron in a high enough quantity to promote algae. (I dose iron fertilizer in my planted tank, but only a drop or two a week.)
Since you said your algae problem is bad, you could try mixing half tap and half RO. That is the highest ratio I would mix - in other words, I would not use more than 50% RO water in the tank. The 50% of tap water will supply your needed electrolytes.
If this half-and-half mixture is not working for you, and algae still returns, then go with all RO water. Follow the other advice also to use an additive like RO Right. I don't know which is better - liquid or powder, since I never used it. I do use a powdered buffer from the Seachem line and it dissolves easily.
Apocolypse Gold adds baking soda to increase carbonate hardness since he has a planted tank and injects CO2. (Correct me if I'm wrong!
) I also have a planted tank and add CO2 but instead of baking soda I use the Seachem buffer that I mentioned, which I think is pretty much baking soda. Increasing the carbonate hardness in this way stabilizes the pH, which is healthy for the fish - they need a stable pH. If you don't inject CO2, ghostie, you may not need to add an additional buffer like baking soda. But you should test your mixed RO water and the RO Right for carbonate hardness, or ask your lfs to do it. For a non-planted tank, I think a carbonate hardness (KH) value of about 4 is fine. This will keep your pH stable. A lower KH value will probably cause pH swings, which aren't good for fish. If you see your pH is getting too low during the week, you may want to supplement the RO Right with some baking soda like Apocolypse does. The more heavily stocked you are, the more you may need to add the baking soda. If your KH test result of your RO Right water is 4 or above already, then you probably don't have to worry about baking soda.
Here's a good KH and GH test kit:
http://www.aquariumguys.com/hardnesstest.html