Note the absence of any finned, swimming, creeping, crawling critters in that tank? Coral only, so not much of a bioload, hence LR not as critical?
Anyone ever coated an overflow or glass with sand using glue or something? I've often considered that when I actually start a tank with an overflow. Cover the acrylic with sand, maybe few shells or other odds & ends. You could try it on the PVC as well?