This might sound like common sense, but... It is surprising how many reef/saltwater will use salt water to replace the water volume lost due to evaporation and then wonder why the sality is out of sorts and increasing. The only elements that evaporated is H2O, the other disvolved minerals/chemicals/elments are stay behind and increase the overall hardness of the water.

I agree with Josh, the water you first use to fil the waterfall really does not matter. But, it is a good idea to get into the habit of always using a declorinator on any water that you use that comes in contact with you critters. If you are using live plants, the plants will take up alot of the disolved minerals, so again declorinated tap water will be fine for top off water. However, if you are not using live plants, then I would suggest using filtered or RO water, otherwise you will see the hardness of your water increase, which could have a adverse affect on your creatures.