I use the Salfert (sp?) they are generally good, the first one was bad. I took it back and exchanged it for a new one. I'd say your test kit with the higher results is correct. I tested my kit by testing water dripping right out of my CA reactor and it still read 320. You could make a super consentrated test with tank water and your additive. I bet it'll come out at what ever it was topping out at before.
Dave
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