I'm not really sure what you mean by grains...but you want to have knowledge of nitrates within the system. You want to try to keep nitrates as low as possible, this can be hard when using tap water as it easily comes in this way.
The other issue with tap water comes from the things we put in that is good for us, but not for our scaled friends. These things easily build up over time and since we don't test for things like fluoride, when we have a tank crash we never end up figuring out what the cause was. As it stands, I've heard of many tanks started with tap water and all but ONE has crashed. That one is currently being maintained by someone I consider one of the most experienced reefers here...and even he has openly stated he wishes that he could get that tank to be run with ro