Ph

My ph is at 8.4 when I test between 6-9pm...is this a problem and how do I fix this?

 

No, it isn't a problem as you pH values are going to fluxuate from day to nightime hours. Your pH is fine, however, if your pH drops, you may want to add kalkwasser, set up a reverse daylight system on a refugium, increase aeration towards the top of your tank at the air/water interface. Basically, you want to decrease the amount of CO2 in your water column!
If your pH is too high, stop using Kalk, if in fact you are adding it, and increase the CO2 in your water. Adding acetic acid(vinegar) to your water will help to raise the pH level of your water as will, efficient skimming, or add a pH controller to inject CO2 to your water.
Also remember that a high alkalinity maintains a high pH! Look at calcium level as well.