use prime for a dechlor and yes you need to add a dechlor when you're adding new water. For clarity, i've kept many different substrates before and when it came down the how clear my water is, they all appeared the same. The only time it would change is if i added wood/peat, or something similar that would stain the water. Now of course when you disturb the substrate, the water will be cloudly but that's with most substrate and would clear up within hours. While the soil itself can play a big part in your success, other factors like light, co2, and the chemistry of your water is also important. People have had success using many things from ada soil to cat litter. Have you had issues with unclear water in the past?