That would explain it. Your tank is WAY too small for the hippo, the emperor, and is even too small for the yellow tank. You also have lots of damsels (clowns are also in this family), which are known to be very agressive and territorial. In addition, you are adding WAY TOO MANY FISH WAY TOO FAST.
At this point, I think it would be to your benefit to rehome the angel once it recovers sonce these are not suitable for anything less than a 200 gallon tank. Do not add any more fish to your tank until it hits at least the 6 month mark, an even then stick to 1 or 2 small fish like gobies. If you have not rehomed the angel by then, you are already way past your limit on fish and should not add anything else.
At this point, I think it would be to your benefit to rehome the angel once it recovers sonce these are not suitable for anything less than a 200 gallon tank. Do not add any more fish to your tank until it hits at least the 6 month mark, an even then stick to 1 or 2 small fish like gobies. If you have not rehomed the angel by then, you are already way past your limit on fish and should not add anything else.