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Potassium is found in all living cells. I you are feeding your tank either prepared or frozen food, you are dosing potassium.

Since potassium test kits are notoriously inaccurate, the logical way to dose potassium would be to keep up with regular water changes. This would bring K up if it is too low, and reduce K if too much K were being introduced by feedings.

The exception would be if it is proved that a given salt mix has a lower K concentration than sea water does. In that case, adding a potassium supplement (for example KCL or K2SO4) to water change water in an amount calculated to bring the K up to sea water levels might make sense.