Diatoms (brown algae) are a good thing they mean your tank is healthy. EVERY new tank goes through a diatom bloom. I would be concerned if you didn't get this algae. As for the water, your LFS might be right, you might be okay using local water. I suggest you test this water for phosphates and nitrates, both of which are food for algae. You could and probably should test for other harmful things if phosphates and nitrates are low (close to zero) but I'd start there. You could also see if the LFS has a TDS meter (total dissolved solids) that you can borrow to test this water. In RO/DI water there is virtually nothing in the water.
It's been said time and time again on here, chemicals are a quick fix but getting down to the root of the problem will help in the long run.
Originally Posted by jacksonpt
your tank is also still very young, so it may still be cycling. Give us a rundown of your tank (amount of LR/LS, filters/media, fish, corals, lighting, etc.) and we might be able to offer more advice.