i have 11 tiger barbs in my 150 gallon tank along with 3 large ciger sharks 8"-13" and the tiger barbs are attacking the cigar sharks fins, i knew they did this but i was told if i kept a showl of 10 or more then they should be ok, but there not.
what can i do to stop this?

many thanks

As you already mentioned, tiger barbs are usually not so nippy when kept in large groups, they are normally too interested in each other to bother anyone else.

If you dont want to rehome them or the sharks, then maybe breaking up the line of sight in the tank might help. Plants are a good way of doing this.

You could try taking the barbs out, re-arranging the tank decor, then placing them back in, not sure if it would help, but you could give it a try.

my shoel of 10 barbs went for 20+cm fish when they where introduced It was heavily planted back then two

I am afraid they are aggressive little fish which i feel do best on their own