is Potassium required for reef???? Help??

[mandarin man]

Thank you guys for helping me. I am new to this amazing hobby and I am trying to learn as much as I can.

I bought the KZ potassium test kit, tested the water and it is almost impossible to see the results without placing the test vial underneath the sun light or a very very bright light. It came out to be approximately 250mg/l to 275mg/l. Any one experiences with this test kit and have a better way of testing this? Am I doing it correctly by placing it under the sun?

I also bought reef pure potassium and started dosing it slowly. Let's see what happen

Thanks