I worked it out to be about 23.4 US gallons, but we won't quibble about a couple of gallons here and there
I agree with Wilder about overstocking. One problem with a new tank (and a new person to the hobby, no slight on you *Sonic*) is that a tank can be set up with multiple problems (for instance, an overstocked tank, inadequate filtration, not cycling the tank properly first, an inappropriate mix of fish) that don't show up right away.
This gives rise to an unfortunate consequence, which is
the belief that everything is fine. After this, since the problems aren't addressed, they slowly get worse over time. Fish are tolerant of a wide array of problems, but sadly not severe problems and not all at once-- it's just too much stress. In the new tank, eventually the problems reach a critical point simultaneously, and become catastrophic, and there are outbreaks of disease, death, unsociable behavior, and so forth.
The best solution is prevention, but since this doesn't address the current state of affairs, the best solution is to
reduce stress. This can be done by choosing which species you'd really like to have (ideally those that are a good fit for your tank size), rehoming species that aren't compatible with this new aquarium plan, and doing regular water changes. You also unfortunately have to do a bit of diagnostics in terms of figuring out if there are other sources of stress (temperature mismatch, noxious vapors being emitted near the tank, overfeeding, etc etc).
Sonic, none of this is really practical advice for you-- I am very sorry to hear about your troubles. Wilder made an excellent suggestion about rehoming the angels, clowns, and shark. I'd also recommend keeping up with water testing and for sure with water changes as noted above. HTH, please keep us posted~