what can i buy to cure tap water

Depends on what you're wanting to remove. Chlorine will gas off under agitation, and chloramines can be broken down and bonded using an additive such as Prime or Amquel. For most everything else--not really. There are lots of products that claim to do this, but really, all they do is bind the matter into a precipitate that settles on the bottom--the 'bad' stuff is still in the tank. Filtering the water in advance, particularly using reverse osmosis filters, is your best bet. What are you trying to remove?

Chlorine/chloramines. Chlorine can be removed by agitating the water overnight, chloramnes must be chemically treated with a product that also binds the resulting ammonia or removed via RO filtration.

Phosphates. High levels of phosphates contribute to algae problems.

Nitrates/Ammonia compounds.

Heavy metals, especially copper.