Why you NEED RO/DI!!!

[theredben]

I was just pondering over the last couple of days if it is sooooo bad to use regular water for water changes and only use ro/di for evaporation replacement. I was thinking that the only reason that MOST people see a problem is that they are using 20ppm water to replace the 0 ppm water that evaporated. This would be an upward trend that would end up with a bad case of algea.