Looking at your photos your reef looks pretty clean. I have never heard of burnt tips on acros from having a dKH of 9-10. I have had burnt tips from a rapid increase in dKH but never at a sitting point at 9-10.
Do a few searches for alk levels in ULNS systems. I experience the issues myself, so I can attest to the 100's of accounts of keeping alk below 8.5 for ULNS systems. If his phosphates are 0.02 and nitrates at 0, it is verging on ULNS. I keep mine at 7.8-8.1 and get great color. There is even some research on it, it is hypothesized that the alk level supports a skeletal growth rate faster then the nutrients can support for tissue growth rate.
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