No one has mentioned this yet, but as you only have a 55 gallon tank your tank is too small for your yellow tang. They need room to "run". i don't know how big it is now, a juvenile might be okay in a small tank but once it gets bigger it will not do well in your tank. I know we were told anywhere from a minimum 75-100 gallons before any tangs should be put in.
Lighting and the type of lights you have are very important. Try to find out what you have and post that info as well. Test strips are unreliable and you really need the kits as others have mentioned. You did not list what your CUC is either. If you are feeding as often as twice a day you best have a really good CUC in there as well. I think to get your tank looking nice you need to get back to basics and start doing some research on what you have in your tank, what they need as individual animals and go from there. This place is a great resource but there are other great options as well, like some great books on the subject that can help you get your tank in optimal condition.