I have a 240 gallon FOWLR tank. I originally purchased a Spectrapure RO/DI system but returned it because it looked hideous in attached to my kitchen sink. The guy at the LFS told me that tap water should be fine being that I only have a FOWLR tank. (Keep in mind I have a wet/dry filtration system (sump), and I have two 40W UV lights). Ok so I filled the tank up with tap water, cycled it for a few weeks. Put fish in there and everything is great! Now my question is if I do a 30% water change will it completely throw off the water chemistry causing my $1000s' of dollars of livestock to die? Of am I just being paranoid? My tank consists of Triggers, Tang, Angels, Wrasses..