Sick cherry barb, please help

[Sam_woolner]

Hi, I have a bit of an issue with a sick fish and can't seem to find what's wrong online.

Background:
165 litre aqua one ar850
Normal tank filter and external tetratec ex700
27 degrees
Sand substrate
Ph6.8
Cl 0
Kh 6
Gh 8
No2 0
No3 0
Established for 18 months
Well planted and large bogwood in the tank.

10 guppies
7 mollies
4 neon tetra
8 cherry barbs
4 black neon tetra
1 bristle nose plec
4 shrimps
3 zebra danios
Some fry
4 corydoras

I recently added my cherry barbs and after a week, one of them seems to have what looks like white string or a white worm hanging out of its gills. Can anyone help with what this is and what to do about it as I'm a bit worried about the rest of the fish.

 

[Samzter]

It probably has internal parasites . I don't know anything about curing and medicating but it could be planaria.