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What would you guys say is the best water conditioner?

Well it depends on what's available to you, or what your certain needs are (eg. waste degrader, odour reducer etc.), but many people use Zoo-Med ReptiSafe, it's a good product, eliminates chlorine & chloramine, and does a few other beneficial things too (waste degrader is NOT one of them unfortunetely). It is also common in many pet stores, the large pet chains will carry this product 99% of the time (unless sold out or otherwise).

Could I use both Exo-terra's Biotize along with ZooMed Reptisafe?

Yes.

Thats what we use with our turtles.