Tap was is a no-no in a reef aquarium. Municipal water treatment centers are known to add phosphate to the water to limit calcification in the pipework and pumps.
If you plan on keeping corals and inverts, you should strongly consider a RO/DI filter. A good system can be had for between $100 and $200, and it will last a lifetime (of course, there's the prefilter that have to be periodically replaced).
You can implement all the good ideas for limiting/controlling algae growth and still have algae. The problem is likely in your source water. As long as you are topping-off, you are adding algae food to your water.
When I was in university, I used to get my RO water from a grocery store every week. After a year of algae problems, I had a friend take a sample to the lab at school. The TDS of the water was obscene (120). As it turns out, they didn't service the filter at the grocery store very often. I finally bit the bullet and ordered a bare-bones RO unit. The algae problem cleaned up after a few weeks of diligent cleaning, and new, clean water.
I now use a 5 stage 100gpd RO/DI filter with an inline TDS meter. Once the TDS goes above 0, I replace my DI resins. If the pressure drops off, I replace my prefilters. That particular filter was $250, but it was by far the best investment I've made for my aquariums. Hands down.
Consider it.
-Rob
Has anyone purchased a RO/DI filter for a good deal? If so, post it!