IMO I would go with at least RO if not RO/DI. I work for a water municipality and I know there are some things in tap that the conditioner will not get out. Heavy metals and other heavier compounds. It doesnt get rid of ammonia and nitrates either. Just locks them up. I would rather not use that if it was me. JMO
RO/DI is the best out there. This is my RO/DI. It`s a bit pricier but you can go on ebay and get them for about a hundred dollars. I just posted this one so you can see what they do. It will be the best investment in your SW tank.