Is tap water okay for water changes???

I use dechlorinated tap water for my top offs. Seems to work just fine.

All you need to test for tap water is nitrates and phosphates, since those are plant food (makes algae grow fast, and not the pretty kind)

At your local fish store you can by test kits with instructions that usually test nitrates, nitrites, ph, and ammonia since those are generally the most important ones but you can order one for other tests ( like phosphates). They give you instructions

You don't have to test for phosphates for the water to be useable, it's just a matter of how much algae growth you'll get. With a tang in there, you probably won't have too much trouble. The longer you let the mixed saltwater sit out (a week is probabvly ideal) the less you have to worry about contaminants from your tap water.

However, your water company is required by law to list contaminants in the water once a year and make it available to all users of the water system. If need be, call them and ask what you need to do to get the report. If the report doesn't hold any really scary numbers, you're probably fine. If you are worried, you can always use RO water from someplace like WalMart at about $0.39 - $0.99 per gallon. Or you could get your own RO system for $80-$200 that removes pretty much all elements from the water except H2O.