Amy,
These are the answers I would have expected for the system with the look of the corals and the open discussion so far. Your biggest issue comes down to heavy nutrient issues in the system, not so much causing problems with an overfertilization of the zoox
per se, but more with an issue of the types of corals you're keeping failing to thrive under the neavy nutrient load available to all the creatures in the water column for the biotope you've created. With the amount of skimmate you're producing at the thickness and consistency your skimmer is producing, you're fortunate that your sytem doesn't have problems with algal blooms of one sort or another, most likely a tribute to the young sand bed acting as a
phosphate sponge for phosphate as it becomes available (something to consider about a year or two down the road as an item of maintenance to change out the sand bed) and top-down algal control by your fishes... Part of this is just maintaining the feeding and cleanup of the number of fish you have in this (55 USG?) system with the corals, part of this is the history of not flushing the rock of its detrital production, and part of it is the sand bed, though not necessarily in the way folks might tend to think (think timing and nutrient sequestering/release cycles rather than just saturtion).
Over a period of time there have been an accumulating pool of nutrients, either in the rock, in the sand, or possibly in a biological sink like microalgal, macroalgal, or bacteriological biomass as an in-system sink. Such pools of nutrients, whether biological (like the algae on or bacteria in the liverock) or physiochemical (like the sand itself in sand bed or the calcareous surfaces of the live rock) become quite leaky over time as they accumulate larger amounts (bigger pools, if you will) of phosphate or nitrate in such closed systems. As they begin to leak more of these nutrients into the water column as conditions change (due to temp, new carbon, blowing the rock after not performing these functions, pH shifts to a more acidic medium as sandbeds mature, or biological production of phosphatases enzymes in aerobic portions of the sand bed), the increasing phosphate levels begin to affect (especially) stony corals in many different ways. Most often we observe a loss of the brilliant coral coloration as the zooxanthellae begin to respond to the presence of phosphate and nitrate by increasing both their numbers and their density in the coral holobiont. We see this as a "browning" of the coral, or a loss of intensity of the photoprotective pigments as the need for these protectants diminishes due to the shading effect of more zoox in the tissue of the host. In addition to this, growth slows in the holobiont as increasing levels of both phosphate and nitrate begin to negatively affect the rate of calcification and skeletalization in stony
coral specimens. This also begins to have a secondary effect on the skeletons of stony corals as the nutrients begin to fertilize certain spp. of algae that grow inside the skeletal crystals (the inorganic part of the skeleton). These are coral bioeroders that turn the coral skeleton green as they are now stimulated to grow (see/google
osteobiosteum spp. and similar genera of marine algae). This double punch to negatively affect coral growth and development can become severe enough to actually cause the loss of specimens as the concentrations of these nutrients/toxicants becomes increasingly more prominent in the aquarium/closed system water column. Your goal now becomes employing methods to drop these nutrient levels and effectively export their presence to reduce their concentrations to levels that do not cause these problems in the corals you keep.
These methods are the normal things we see postings for every day: maximizing exports while minimizing imports of both the raw nutrients and subsances that contain the precursors for these nutrients before they have a chance to decompose in the water column and make these nutrients freely available. Limiting imports wil refer to issues associated with the following:
- top off water (use RO/DI or distilled water whenever possible)
- quality GAC
- avoiding unnecessary use of additives
- limiting feeding
- limiting fish populations (and deciding whether your tanks are "fish" tanks or "coral reef" tanks)
- removal of ALL dead specimens ASAP
- avoiding use of physical filtration (or at least cleaning the filter medium every day or so)
- maintaing rock off the sand beds to allow detrital shedding
- using a quality nitrate- and phosphate-free salt mix
...while maximizing exports will refer to:
- running a high efficiency skimmer and keeping it clean
- regular water changes with healthy amounts of water based on the population densities of the creatures in your system (and the size of your system).
- siphoning out biomass when you do experience bloooms
- blasting the rock to remove detritus
- siphoning nooks and crannies to remove detritus (esp BB systems)
- quality GAC and a good schedule for changing the GAC
- phosphate resins (not for Sarcophyton spp. tanks) or GFO reactors
- appripriate sand bed or BB husbandry (this includes changing the sand bed every several years at saturation)
- removing and replacing calcareous sand as it becomes saturated with phosphate
not necessarily exhaustive lists, but I think you get the idea...
One of the biggest mistakes we make in controlling our little slices of the ocean is to think that we can crowd one more creature into our microcosms to make the system look more "appropriatly full" yet fail to think of how the biological needs of that addition may affect not only inputs but exports of the system as a whole. This then becomes an issue over time as we work out the means of maintaining the unnaturally high population densities with what we can artificially engineer this little biological system to perform. This is not to say that we cannot engineer these systems to handle these loads, but that we often do not see the needs of such systems as limits to what we might easily undertake as opposed to what we end up undertaking in terms of what work inputs we must do to keep the systems looking good and algae-free. Most often, it comes down to deciding how much work you want or are willing to do (to the system as a limit to how many fishes, corals, different species, mixed biotopes you want to interact and curate) as opposed to any real biological limits to such mixes. This is where most aquarists get in over their heads in running their aquaria.
I'm not saying that you're in over your head by any means, but that there is a good deal of work that must be done to keep systems with lots of fishes and corals and other miscellaneous creatures in the system running at their most beautiful and brialliant coloration and rapid growth compared to systems with just a fish or two but many corals. The bottom line then becomes deciding how much work do you want to do with a very needy biological system, or deciding to go with a less complex specialized (single biotope) system with fewer creatures that is easier to control.
You currently have a very needy system...
меня зовут Chris 
