The best way to do this is to keep it in quarantine in a separate, smaller tank for about a week before introducing it to your other frogs. You can get fairly cheap cover-equipped plastic housing units of various sizes for such purposes. These are also really good for use when transporting frogs. Before you add that water to your frog tank, make sure it has been de-chlorinated. Best procedure here is to get a little bottle of dechlorinating solution from the nearest pet store.
Stale water is always much safer than straight tap water.
Once this is done, clean the water regularly, the way you would with fish. It may be a good idea to keep one of those pH testing kits around. And finally, don't overfeed. However, frogs need well ventilated tanks, particularly those that end up needing greenhouse-like environments. The moisture in the tank can lead to all kinds of fungal and bacterial infections.