Good to know that you have done some research.
I'm not really an expert here, so my 2 cents worth...
1. Tangs like your blue tangs and sailfin tangs will grow rapidly and require lots of swimming space. That's why they are normally recommended to be kept in at least 4 ft tanks and above.
2. Even if your tangs seem to have enough swimming space, it doesn't mean they are not stressed as these fishes travel miles in the ocean everyday. Not to mention you have 3 of them squeezed in a 2ft tank, they will be fighting for territory. Highly stressed environment can result in many things, from torn fins to white spot outbreak in your fishes.
3. As peacemaker has mentioned, your tank's pretty new, so it will lack pods for your mandarin fish to feed on. Mandarins are known to be finicky feeders, and most don't wean to processed food. What this means is your mandarin is probably going to starve to death unless it is already feeding on processed food.
4. Your clownfish sounds like it died from Brooklynella. Common in clownfish and can spread to your other fishes. Suggest Formalin dips for your fishes. Pls do a search on the forum first to find out the ABCs.
hope it helps!