You need to quarantine any new fish for about a month before adding them to your tank. Once you are sure they are healthy add them a few at a time over a period of a few weeks. Doing both these steps should help ensure that this same problem doesn't happen again. And I would strongly recommend purchasing a drop test kit for ammonia, nitrite, and nitrate - without it you really have no idea what is going on with the chemistry in your tank
And yes, your tank could easily be overstocked as you didn't mention the size of the angels nor the type of pleco you have. Common plecos, even young ones, produce alot of waste and get huge, and adult angels are quite large and will probably outgrow the 20g tank.
Cories are schooling fish and should be kept in groups. I would consider getting more the of same type before adding anything else. Or rehome the one that is remaining.
Last edited by wendyjo; 11-23-2011 at 12:06 AM.