One of my tangs ripped through 8 cleaner shrimp I had in my 125g, not to eat them but to just kill them. Tangs are aggressive and should not in any circumstances be in a 30g tank. The angel is also and aggressive fish so that pretty much explains what happened. \
The added bioload of the two fish could also be the culprit, I suspect between that and the aggressive fish you now have this issue.
And as a side note, 3 clowns in a 30g is a big no no... I had 2 pairs in a 6 foot tank and they were fine for months until one day the one pair tried to kill the other two, luckily I was home and had an extra tank to put them in.
Maybe in a 220g or bigger you can have multiple clowns and some people have had some luck doing it in smaller tanks but it will almost always result in disaster.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.