30liters is about 8gal so yes I'd say your way overstocked. If you mean silver-tipped catfish .. they get HUGH .. up to 12-13+ inches is more than likely.
The L. Danios (I'm assuming your meant Danios) are very active and need a larger tank volume. Both Neons do better in a 10gal min (40liters)
To keep your current stock (I'm assuming they are juveniles and are still growing) you're looking at a 120gal (400+Liters) easily. Get rid of the Silvertips, and a 30gal (120Liters) should do.
Keeping that many in your tank will probably lead to stunted growth.