Clean your tank and do a water change. Keep what you have for now. Let your tank have some more time to adjust to your bioload. You added way too much to your tank too quickly. Your bio system can take months to properly mature and once it does always add fish slowly so that your system can adjust...
Your biosystem is composed of nitrobacter bacteria among other things... This bacteria lives in low oxygen areas of the tank including any live rock and in the substrate. When you add fish this bacteria colony has to have time to multiply to balance out the new load. You obviously overstocked your immature tank...
Start slowly. Look at getting an ultra violet sterilizer. I have one and have NEVER had a fish illness since. Since you may be having disease problems, always quarantine your new fish for about a month before you put them in the tank. Parasites often have to find a new host or they will die. If you let the fish stay by himself for awhile in quarantine then the parasites that may be on him will die off and then you can put him into your tank.
Your nitrates are at 40ppm... Ideally they should be at 0. This is an indication of having too many fish/not enough filtration. Invest in some live rock and you will improve your biological filtration.
If you feel you need to medicate your fish, NEVER do it in the main tank. Buy a 10 gallon tank and use it as a hospital tank to dose your fish. Read up on ICH in the disease forum. There are lots of pointers there...
Basically... slow down. Do proper maintenance procedures including water changes and only use the best filtered water you can find (reverse osmosis/deionised).
One thing I have learned to break aggression in fish is to rearrange the tank. I had a yellow tang that was very territorial and did the same things. I rearranged the entire tank and completely reformed his territory. Then I left the lights off for 24 hours so that it would reduce the stress in the tank. After that he found a new territory and left my other fish alone.
Also you need to watch "stress" in your tank. Fast movements and constantly messing with the tank will cause stress in the fish. Stress quickly reduces their immune systems and it leads to diseases such as ICH. Some of the best ways to reduce stress are to leave the lights off when you are doing anything major to the tank and reduce feeding... this will slow down their metabolism and will help them get through any tough times. Always be easy on your fish. You can literally scare them to death.
