Sounds like you have a case of columnaris running rampant in your tank. I'm sorry to say I'm not surprised; your tank is really overstocked. Angels alone need around 10g each. The stress of all those fish, the strain on the filtration and what sounds like infrequent water changes are likely reducing the immune system of the fish and leaving them open to disease.
I strongly suggest upping your water change frequency; at least once a week if not twice (around 20-30%); also, what are the ammonia levels? I also suggest looking into another large tank; if thats impossible, you may want to look into returning some of the angelfish or giving them away/selling them.
Lastly, I recommend reading the following article. It give great advice on what columnaris is: http://www.aquamaniacs.net/flex.html
. The only thing I'd add to it, is if the fish aren't eating, use Kanacyn, If they are definitely get medicated food; you can find some here: http://www.flguppiesplus.com/_wsn/page3.html
aka Cycling Guru and the Ich Slayer
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