Mimic or Mustard Tangs

[saltypatti]

Is the mimic or mustard tang just as onry as the yellow tang??? I just lost a copperband butterfly today. I don't think that it was because of the mimic though, but the mimic did pick on it a lot. Lately it has not been picking on the BF. I think that it was not eating or I had less than perfect water quality. How do you maintain good water quality without breaking the bank??? I know that I need a protein skimmer. That is on the purchase list. I also need a better filter.
I have a 55 gallon tank. I have plans for a 125 to a 175 so I can enjoy more fish and inverts. For awhile, all I had was a bunch of live rock and no fish. Kind of ugly. How many fish can you have in a 55 gallon???
I had a variety of five kinds of food that I was feeding and I never saw it eat any of that. I only saw it eat off of the live rock. I am hoping it did not starve to death. I noticed that it started leaning to one side and then just stop swimming and laid on the sand or the rocks. That was my daughter's fish and it was hard to let it go. She is only 6 years old. But it is part of life. I have 6 fish and they are kind of small. Do I have too many in this tank????
What is the purpose in having a tank if you can not have a good number of fish??? I have had this tank for 11 months.
I want to see this fish live a long healthy life. Any suggestions are helpful.

 

[saltypatti]

Here is what I have:
I have a 55 gallon with basic filtration, no protein skimmer which is next on the purchase list. I also do not have a QT which I will invest in before I consider any other inhabitants.
The tank has been up and running for 11 months. I just checked the water quality yesterday.
ph is 7.8
ammonia 0
nitrites 0
nitrates 5
This was before my routine water change. I also need to get a thermometer.
I have the following in the tank:
60 pounds of live sand
70 pounds of live rock
a pajama cardinal (11 months)
tomato clown (4 months)
yellow tail damsel (4 months)
coral beauty(2 months)
tile starfish(3 months)
horseshoe crab(11 months)
mustard tang(3 months)
spotted mandarin (2 weeks)
I have lost in the past month the copperband, a sixline wrasse a seargant major and a blue linkia.
I also had a orange linkia for 11 months and it died when the blue linkia did. It looked like it lost it's intestines and was either deteriorating or being eaten on.
So far the tile is having the same problem, but he is recovering and doing well.
I am fairly new to the hobby and learning a lot of things the hard way as I am sure most of all have done. I have talked with a couple of people who have a tank my size and it is stocked with a lot of fish, corals, clams, anenomes etc. They hardly ever had any fish loss!
I am definitely considering a much larger tank, but I want to get much better equipment and I want help to develop a really pretty aquarium. I want better lights and I do want to have corals and mushrooms. I guess the copperband is not a good fish to have. I may consider it when I can get the other tank and use the small one for the small peaceful fish and make it a fish only tank.
Most of all the fish are eating really well, but I only saw the copperband nipping at the rock. She may have eaten the feather dusters, but would not eat any food I put out.
Is this tank ok the way it is. Can I leave the current inhabitants and not add anymore, just better equipment?? Can I add the corals and mushrooms??? I will get the skimmer and lights first though. I would actually like to have less live rock. All I see is live rock and the fish hide alot. I added fake plants with color because it looked drab. Can I send you a picture and get some ideas on how to make the tank better???
I really think that the copperband fell prey to agression, diet, stress, and poor diet. It was probably a combination of all together.
One final question, what is the most humane way to euthanize a fish. I can not stand to see it lay on the bottom of the tank and breathe heavy. If I take it out, it will suffocate too. Yesterday, I burried the CB in the sand, and although she tried to come out a few times, she died pretty quick. It was hard for me and my daughter. I did not know what to do. I should have taken her out and put her in another tank and she would have probably been fine, but I did not have another tank to put her in. It was already too late and their was not much I could do. These are some heavy prices to pay, financially, but emotionally more.
I will most definitely invest in the two recommended books and hope for a lot more success. Thanks for your help and suggestions.