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I started my tank with tap water and am having ro/di water issues now - I wouldn't skimp on a RO/DI unit. It'll save you tons of aggrevation. I've put thousands into my system and it looks awful...just over-run with green and hair algae.
Clean water is inexpensive and makes a huge difference (of course, depending on your tap water quality), especially with a small tank. The filters will last a long time.