Potassium concentration in natural seawater is approximately 400 ppm. Many aquarists have reported that a low potassium value reduces colors in corals. This can be corrected by increasing the potassium value to the normal natural concentration. You can accurately measure this value with the Salifert Potassium Reef Test kit.
This kit has a resolution of 15 ppm, sharp color change, can be performed in about 90 seconds and offers value for money.