How long have you had the tank set up?
Do you test the water at all, i think a test kit is a must to be honest or atleast take a sample of the water so they can test it at the fish shop, that way you can tell if anything is wrong
30 litres is really way too small for the tiger barbs
Aside from the small tank .....Tiger barbs are fairly sensitive fish when it comes to water quality. How often do you do water changes? I would say your tank is fairly overstocked and frequent water changes would be ideal to keep atleast a stable enviroment for them. Do a water change and see if that helps....also next time you get your water tested take special attention to your Nitrite levels as Tiger's dont cope well with it.
id do 50% water changes (some declorinator is a must if you havent got any) twice a week and feed only once a day untill you get ur water tested and has and the results for ammonia and nitrite are both 0
Also save up for a bigger tank to house those barbs