Depends on where you ALK is. If both are high you run the risk of cloudy water or scaling if the pH goes high. The general consensus is that calcium is not needed in a fiberglass pool but I have been finding some conflicting info with no hard data to back it up one way or the other.....Fiberglass is more prone to staining by metals and perhaps the higher calcium levels help prevent this. Don't really know. Mbar might have some input on this subject. Maybe she will see this thread and reply.