Originally Posted by bettababy
280 is way too low for calcium. The animals, fish, inverts, corals... all need enough calcium for growth and organ function. Ideally calcium should read between 400 - 450. Without the proper amount of calcium in the tank, you are going to have many different kinds of issues, such as death, sick fish, dead/dying inverts...
And, as I mentioned in my last post, that is too many fish in that size of a tank. Waste issues are going to be a constant issue, even with daily water changes it will be more than the tank can handle, which is already apparent in your nitrate (no3) reading. Either a bigger tank or fewer fish is the answer to that.
I was asked for my input on this... sorry to take so long to respond, I just got home from Jamaica.
My initial reaction as I began reading this thread was that low calcium levels, high nitrates, and insufficient aquarium size have resulted in a weakened immunity over a period of time. The scratching behavior is the visual evidence for this. Reading this entire thread, I agree with Dawn word for word on her posts, and would have been posting almost identical responses.
The fact that this Yellow Tang was purchased at a 2'' size and looks the same today as when purchased is a big hint that the environment is to small. This is a fairly fast growing fish and should look much larger with more girth. Tangs growth to 70% of their adult size within the first year of their life, and placing them in a smaller sized aquarium stunts this growth, resulting in internal development issues that inevitably leads to a shortened life. I personally would suggest an aquarium of 6' in length would be wise for any Zebrasoma species of Tang, although I have seen some success in aquariums as small as 75 gallons.
For the record, there are some other things on this thread that I would not initially be as concerned about. I do not necessarily see a problem with using tap water on a fish only or FOWLR aquarium. Tap water in different parts of the world (or country) is treated differently, and an actual analysis of the tap water being used would be needed to have a good discussion on this. For the sake of argument, however, I will suggest that calcium levels under 300ppm could be an indication that something is going on related to the source water, be it the tap water or the salt mix.
I also would want to dig much further into the conversation about live rock absorbing phosphates. I have been at this hobby a very long time and can't recall seeing anything on this portion of the discussion, not in magazines, books, forums, chat rooms, or trade shows. Unfortunately, I have followed the posts of the member (reefsahoy) bringing this topic up, and I have had great respect for his (her?) experience in this hobby and obvious knowledge base. I am confident that there is information in this part of the discussion that will be beneficial. Reefsahoy
.... are you looking towards phosphates as a potential cause for the rapid calcium depletion?
[We are getting off topic with this I think, and perhaps getting into a very advanced discussion. As a moderator I would like to keep this very brief, or invite you to open a new thread where we can discuss this specific issue. There might be something here that a number of us can learn from. If you do open a new thread, please put a link to it on this thread.]
As a final note on this issue, I agree that pictures of the fish and overall aquarium would be extremely valuable. I can usually gather more helpful information from pictures and water parameters than I can from a conversation, especially when trying to help diagnose a disease or other potential problem.