Where to start?
A fourteen gallon biocube is going to keep you limited on fish and by limited i dont mean the amount of fish either. it also includes fish that need swimming space and a 14 gallon biocube wont give them that space. Your corals and inverts are doing fine because size doesnt bother them as much. the purple dottyback and the clown were a smart choice to add because they dont need a lot of space for swimming and i would only say you can add one more fish, for example another clownfish.
They did well for 2 weeks and I added a hippo tang and a mandrin dragonet. Everything went south from there.
Well there are a lot of problems here. You just recently added a clownfish and dottyback. everytime you add a new specimen to the tank the aerobic and anaerobic organisms need time to react to the change in the bioload and other reactions. This is extremely important and is why i stress stress stressssss patience! patience is key in this hobby. You shouldnt add another fish to your tank for a month/ month and a half after adding a fish.
: youre obviously new to this and these are not easy fish to keep. i would stay away from them they need pods
to eat which naturally grow in your tank and since your tank is farely new i would not get one. Also a 14 gallon tank isnt going to be able to supply enough pods
for the fella and it would be like a starvation camp to him.
: Absolutely Absolutely Absolutely do not put any kind of tang in anything smaller than a 75 gallon tank. These fish need an extrmely large swimming area and they do not survive well in small tanks. it will outgrow the tank quickly and its just not right. Its like cruelty to them, but obviously you learn from your mistakes. Please stay away from Tangs, Angels and other large fish..
My lfs said try a cleaner wrasse and a cleaner shrimp.
Wrasses are very active fish and they too also need to be in large tanks. stay away from them as well. i doubt it was a poor specimen but i wasnt there so im not going to argue you. Without the proper amount of water a fishes health can deteriorate quickly. The cleaner shrimp on the other hand will be fine which is a plus.
And heres the verdict...
the culprit is your LFS
and you.. research is key, im sorry i said it but its true. you need to research more, there are so many little things that could go wrong with putting certain specimen with others. Dont get discouraged... simply get rid of the arrow crab they can get aggressive and try starting with one clownfish and let him live for a month by himself.. then add another clown and see how that goes. the reason the dottyback got ich was because of the living space... he fealt threatened and didnt have his own territory which stresses fish out