Yup, no meds for it.
Two primary issues we keep asking about are water chems and feeding. You may think they are great and you know what, they may be. Allow yourself the pat on the back and put people at ease. When addressing disease issues, people want to see the readings. A classic example is people wondering why their inverts die and they have "perfect" specific gravity. Upon further investigation they do, at 1.020 it is, many would argue, "perfect" for fish only systems but fatal to inverts. Nitrate levels (in particular though I imagine chronic poor water quality including pH issues and alkalinity could be a concern) chronically above, lets say 60 would be a concern.
Tangs and angelfish require lots of diversity in feeding, lots of frozen formulations for herbivores/omnivores and angels in particular...home made food, lots of greens (esp nori with less stuff like romaine)...and less so in pellets/flakes if that is part of the diet. Also brine should not be a primary diet. Additional vitamin supplementation by soaking food in selcon or similar - or a variety of other products now on the market helps.
You can replace everything peice of equipment on a tank, and still have stray voltage...that is something to test for and eliminate (though many eliminate it offhand, I think it should still be considered).
Really people do want to help. If you can't get a larger tank, at least consider making sure everything else truly is the best you can provide. Even if you had a rectangular tank...it is better than a hex. Hex tanks tend to be tall, and thus have a number of issues with running properly (in particular, gas exchange and pH fluctuations). So though it is no where near ideal, if you can find a 3 or 4' space for a 58/65 or 55g it would be significantly better, IMO, than a hex...though not ideal.