Your tangs are going to expect stable water. With rocks coming in the water will not be very stable for the fishes for sure. One wonders if it is stable now.
The red/purple you see is indicative of the instability of the system, possibly too much phosphate. Have you checked for organophosphates?
Another point of your system: There won't be much room for corals and the fishes after the next rock addition. That will be a total of about 230 lbs of live rock in a system of about 150 gallons. I assume your aquarium is in the neighborhood of about 135 gallons or less? plus 75 pounds of substrate. That isn't much room left for swimming and the tangs need
a lot of swimming room.
Thus, in addition to stability issues, your tank isn't large enough for those tangs you currently keep. You'd need a tank starting with the standard dimensions of a 180 gallon aquarium, but if you want to keep both, you should consider a 240 gallon.
Both the above are stress issues on the fishes.
I'd suggest getting rid of the crabs. But. . .that's another story/issue.
The above issues can contribute to the fishes sense of well being which manifests itself in weight loss. However, to be conservative I'd perform a de-worming on the fishes.
There are three (3) basic possibilities when it comes to an intestinal problem. This reference outlines the three nicely:
http://www.petsforum.com/personal/trevor-jones/internalinfections.html
Sometimes an internally infected fish (bacteria or worms or parasites) will stop eating or slow its intake. But usually there are other signs, like loss of weight, ravenous eating but no weight gain, and/or strange excrement. Consider the possibilities by reading that article.
Whatever the internal problem, the fish needs the best water quality, best environment (larger tank with more swimming space) and the best diet AND supplements added to its diet.
I think your feedings are pretty good. I'd keep away from too much of the carnivore foods and find ways to increase their spirulina intake.
The drug Praziquantel (a.k.a. droncit) will treat intestinal worms, Maracyn Two for Saltwater fish will treat an internal bacterial infection and Metronidazole will treat dinoflagellate infections.
To be conservative: I recommend this two-step process:
Treat the fish for worms (using Praziquantel) first and if no weight gain in a few weeks, then transfer to a quarantine tank and treat for Dinoflagellates (using a med containing Metronidazole). But I am still unsure if your fish will fully recover, it being in the less-than-optimum environment and all.
Praziquantel is best administered orally and can be done while the fishes remain in your display tank.
Check this out for some guidance on the deworming:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=644238
By excrement 'about right' I meant it is about normal or what I would expect from a tang. Sorry for the choice of words.
Good luck! :rollface:
