Does brita make tap water ok?

[chaozie]

I know tap water isn't good for marine fish tanks but I use it in my freshwater tanks all the time. I just let it sit for awhile and add stress coat and my tanks with their plants and fish have been fine for years so why is it bad for marine tanks. Also I know you can go to the store and buy bottle water for the tanks or buy a ro unit but what about a water purification system like pur or brita, I know there not as good as bottled or ro but would the water from them be ok to use in a tank?