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K balance in a reef tank:

Inputs: food, supplements if any, water added during water changes

Outputs: coral uptake of K, coralline uptake of K, skimmate, water removed during water changes, macroalgae (refugium) pruning

Who knows if K in a reef tank rises or falls over time.

If there is a reliable K assay then I would be interested to see what K levels are in a number of different reef tanks and whether K level decline or rise over long periods of time.

First you would need someone with some decent lab equipment to see how accurate the K test is.

If it is shown that K does tend to drop over time in most reef tanks, then I would consider measuring it and supplementing it.