Fin Rot

[fishiefishfish]

hi my fish have had fin rot about 4 times in the last 4 or so months, particularly my angles. They have it now. Can someone tell me what exactly causes it? I know its the water, i cleaned it out the other day and they got the fin rot after cleaning it. Is it caused by amonia or ph or something like that? Can rotten leaves that have fallen off leaves cause it because i was told to make sure I removed them, a coulple of my plants are losing their leaves.
I have a 200L tank with community tropicals - barbs (not fin nipping ones), angels, guppies and tetras. Any help would be appreciated