Who Gives A Fish Forums

Just setting up my 55 gallon cichlid tank and I've got a question about my substrate and rock. I've picked up 25kg of ocean rock and 16kg of coflake sand - will this combo push my pH too high? I'm planning on keeping lake malawi cichlids, so I need to keep the pH between 7.8-8.6.

It's not going to be a problem.
Ocean Rock and Coflake Sand are only going to help buffer your pH, they won't actually cause it to rise.
You're in the clear as far as that's concerned, your pH should stay within a safe range for your Lake Malawi cichlids.