Thanks to both of you for the prompt reply.
While I'm not going to get into the expense of another tap filter right now, the choices would either be treated tap or purified. I was using purified for some time when we had a serious diatom problem (still do, but to a lesser extent) thinking the tap water was the culprit, but we since went back to using the tap because we just couldn't afford to add gallons of purified whenever doing a water change or top off. The purified didn't seem to make a difference with the diatoms anyway, not eliminating them in the least bit honestly. I added some purfied yet again last night just to top off after picking up a couple of gallons in the supermarket, but I'm beginning to think mixing all these types of waters isn't good for the ecological/biological balance in the tank...
Regardless, wouldn't the purified cut down on the disgusting lime/mineral deposits that calcify all over my filters and the back of the tank? Isn't that coming from our tap water?