One thing that may be part of the swings could simply be poor testing practices little things can easily swing your results a long ways. If you get a funny number always rerun the test. One thing I've found with po4 test is it's very hard to get all the reagent out of the packet. Also entrained bubbles from shakeing the vials will really mess with the result. That's where I'd start and if it's still occurring then I'd look into making changes. If your tank is doing well other wise I'd be tempted just to ignore the numbers and leave it be.
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