Last year I was having problem with my clearing agent and water, I was getting scum (white deposits) on final prints even after proper wash.
What you were experiencing was calcium sulfite being precipitated from hard water. Your solution was a correct one and any calcium chelating agent could be used. I would suggest dissolving the sodium hexametaphospahe first in a small amount of hot water and then adding it to the rest of the solution.

I would also suggest acidifying your clearing bath with sodium bisulfite as in the formula given below.

You could also use the following formula which is very close to Kodak's Hypo Clearing agent.

Distilled water (50C) …………………………………………… 750 ml
Sodium sulfite (anhy) ……………………………………………… 100 g
Sodium bisulfite …………………………………………………………… 25.0 g
EDTA, Na4 ……………………………………………………………………………… 2.0 g
Sodium citrate ………………………………………………………………… 5.0 g
Distilled water to make ………………………………………… 1.0 l