It would be better then tap water, but not by much. I could only imagine using it for a nano reef it would take forever to produce enough water for a larger tank. RO
membranes are so fine that only H2O molecules are allowed through. Brita's use a resin similar to DI
units for removing things that make your tap water taste funny, but that's about it.
Brita replacement filter cost in the arena of $30 to replace. You can find a good RO
unit on ebay for $69.00.
You should also look into other things that could be causing the nitrates to rise....overfeeding, additives...ect.