Hey, so this thing you saw that removes impurities was a carbon filter, I'm assuming. Carbon filters will remove excess ammonia etc. from fish tanks, and they really don't do a heck of a lot besides that, and well, aeration, but that's only because the water's moving.
If I'm correct that it was a filter you saw, then it wont do all that much for your H20-- no more than one of those filters that hooks up to your sink. As someone who's had saltwater fish for a long time, I'd be surprised to find anything that would clean your water and adjust your PH in the way you want it to... sorry to say. They do, however make PH raisers and lower-ers for fish tank H20 that they sell in Wal-Mart and other pet stores, but they're not good for your plants.
Stick to purified H2o with nutes, or do rainwater like skunk said. Good luck!