My fountain is called a Wok Waterfall. It's basically a 36" wok shapped dish that cascades into the pool with a fall of about 10". It is made entirely of fiberglass BUT...I have put 24 pounds of smooth, flat river rock in it mainly just for weight. Probably 24" diameter and 3" deep. I will retest tonight and post the results. Could the rocks be a source of calcium? I don't believe they are very porus. Each is about 2 to 3" round and 1/2" thick.

Donnie