Agree, looks like an infection. As betta fish stated it probably started out as fin nipping and became infected due to stress and possibly poor water quality.
To treat this infection I would first start with a large water change of 50-75%. Then I'd run to the fish store and try finding some Methylene blue. Grab a gallon of water from your main tank and added about 1/4 of a teaspoon of the M. blue to it. Net out all effected fish and add to the bucket. Let them sit in the bath for 15-30 minutes. Then just net them out and put them back in the tank. Do not add the M. blue to the main tank, all sorts of problems will occur.
The minimum recommended tank size for 4 angels is 55gal. It can be done in smaller tanks but you also have a lot of live bearers that create a good amount of waste, as does the pleco. What kind of pleco do you have? A common pleco will need a much larger tank as they reach about 2ft long.
Because you are overstocked your water quality becomes and issue. Can you test the water for nitrites, nitrates, and ammonia then post the results? I've a feeling the nitrates are probably quite high. You'd need to be doing at least twice weekly 50% water changes to keep them down.