OK based on everything that's been said to date, I think the lighting schedule you've been running since November is the #1 likely suspect. With a small tank, you can easily blast too much light down to the bottom of the tank. I'd probably lower your overall intensity even more. I run the radion on my daughter's 20G cube at 50% of the overall intensity that the radions on my DT are at. After a couple weeks at lower intensity, see how things look. If things are going south fast, you can borrow my PAR meter. I'd just ask that you pay for shipping both ways.
PS Really good looking tank. I like your aquascaping and coral placement
PS Really good looking tank. I like your aquascaping and coral placement