At the time I did the HTH hardness test with the weird results (yellow solution to faint green), my pH was low and so was my alkalinity. (I did the test twice with the same result both times - as I swirled the blue reagent the wisps turned red before disappearing.) The numbers at the time were:
pH - 7.0
TCL - 2.5
ALK - 30 ppm

I have since brought the pH up (7.3) and the alkalinity up (90) also. I will repeat the HTH test tonight and see if it still acts weird.

Peter