Has your parameters been in check for the whole 15 months?
I have had a growth spur since I've had my current tank. I rarely check parameters unless I notice something being stressed.
If things aren't working currently then try this,
If your currently at 0no3 and 0po4, try dosing aminos and phyto. Read the recent article in the general discussion forums about their uses. Since dosing NOPOX, ReefEnergy A&B, CA/ALK/MG, and keeping everything in check. I've had great growth. I monitor parameters every so often to check if they change then adjust as necessary. I keep up with husbandry so I don't need to dose often.
Since you said you overfeed your tank/feed your tank well. I would assume you need a boost specifically for corals. That's my reasoning behind the products I suggested.
If you buy from Amazon it's like 30 bucks (ReefEnergy A&B 500ml bottles) so it won't hurt the bank to try. You also want to give it a few weeks (3-4) before you judge it.
Another thing I've noticed with my tank is pH dropping at night when I took my fuge light off for a few nights. They were closing more often and seemed a little more stressed than previous nights. After adding the light back on, they went back to normal.