Saltwater's not all that hard once you get all the kinks out of the learning process. Things die, things collapse, but life goes on. I hate to bring up how many anemones i've gone through before I got it right... That's just how it is. You always learn from your mistakes.
Tank is looking nice. You're getting more live rock right? More LR, and a filter, even a HOB Would do some good at this moment. Don't get any more fish until after your tank is pretty much recycled. Since it hasn't been up that long, you can probably get away with going through another cycle once you get the extra LR. Just keep an eye on that clownfish. Don't add anything(with the exception of a few snails to deal with diatom blooms) until everything zeros out.
could be due to any number of reason's a new tank, a water parameter was out of whack, the fish didn't do well acclimating to your system, ph or temperature drop any number of things could have happened.