It doesn't sound like it was cycled in advance (one piece of live rock just isn't enough), so then when fish were added, ammonia spiked and killed some of them. It is likely ok now, but as the fish mature, it's going to be crowded and overstocked (the unnamed blue fish look to be a damsel, which are nasty tempered even in large tanks). Clowns are in the same family and equally territorial. I'd take the damsels out, and leave just the clown, goby and shrimp.
And the skimmer is working overtime because there are a lot of wastes in the water. Are water changes being done? how often and how much?
Hey guys so real quick background information. I work at a nature center in Weslaco, TX, and we just installed two tanks (fresh & saltwater). Now the other park technician took lead in the tank setup because he used to work at a pet store and he has experience with saltwater tanks. We basically made everything ourselves except for some parts we ordered online (protein skimmer, pump, etc.) and it seems to be working fine, but we keep having to refill the tank with water, i'm pretty sure its due to the loss of water via humidity, I just wanted to know if we should buy some plexiglass and just cover the sump.