Victor, are sandbeds crashing in 6 months now?
He has a buildup of nutrients from overfeeding, IME. Had that happen a few times myself, must be an Alberta thing.
Upgrade the skimmer, deal with the excessive feeding, and you'll be on your way. And avoid that epoxy...
Oh, and for a short-term solution, try Dr Bob's Chemi-Clean. Cleans that stuff up like nobody's business. (Large water change after.) Check for LTA compatibility though... just thought of that.