How long have you had them?

Id say give them more space, a 20L should do with half that in water.

Try live brine and some frozen bloodworms, mine eat everything from fish fry to frozen blood worms. Try that it may help try some live brine, bloodworms, and mysis shrimp, and live bearer fry if accesable. Maybe even pellets depending on how the shop that you got em from kept them, that might be all they are used to.