mmm dont you think that its a bit tooo many in a 20L tank.. got any other tanks. I like 3 males to 2 females in cories. Try them in a 2ft if you can.. leae them in there for 2/3 days..feed them well. On the 4th day do a cold water change (af night, thats when i do them), so drop the temp by 5-8 degrees and then watch what happens.