Yea as Ryan said, the tank is proberly not good to have convicts in it and proberly also the jewel. Convicts are extremely aggressive and will terrorise everything in the tank as they get older and also fairly aggro when little to.
I'm not sure what you meant by the 20 litre tank? have you got the convicts in that? as that will be way to small for them they do pick on there own kind and will cause nothing but grief.
In saying that - I have convicts, but have them separated - males in a 700 litre african tank and females in a 400 litre tank and believe it or not the female tank is the HORROR tank they are so nasty where's the males are quite cruisey. I way prefer the males
I started breeding them so I could feed the babies out to my turtle but the turtle doesn't like them now - so you have to think of that also or else you'll end up with 100's of babies that your fish won't eat and no one will buy off you :-?