gold fish are ammonia factory's you need to check your ammonia and change out water as needed.
120 L = 30 gal
5 new gold fish on top of what you had is quite crowded
I suggest bigger tank , or pond or bring them back , with that many gold fish you will be doing daily water changes for sure .
large amounts of ammonia will deplete the oxygen level in the tank , the fish will start to die off .
this is a very good article that you should read it may help you better understand the problem.
the dangers of over crowding this may help better understand