Water Mistakes?

I was going to begin to use distilled water too, but when I read about it, it didnt seem like a good idea. So i buy some kind of Wal-mart water, says its ro/di instead of using tap water. Its quicker than filling up a bucket with tap then using amaquel or something like it and waiting for it to dissolve. So far, I have no problems taht I know of. I only been doing this for approx. a month now. Also decided a water change once a month instead of once a week is good. Everything is alive, except one coral which I think was dead on arrival. But the mat is still purple so I am keeping it in there as things have grown on/ in it. Also quit on adding calcium since my salt has it and eveytime I test is its about 480-500.