Freshwater Fish Parasite? Planaria?

Freshwater Fish Parasite? Planaria?

Observation - Freshwater Fish Parasite? Planaria? - UK and Ireland. Description: In my freshwater tank I have observed a bloom of worm like creatures in the pebble substrata. From research {google} they don't look like Planaria. The picture is of a parasitic 'thing' 15mm long, that sticks to the side of the glass, but also attaches it

In my freshwater tank I have observed a bloom of worm like creatures in the pebble substrata. From research {google} they don't look like Planaria. The picture is of a parasitic 'thing' 15mm long, that sticks to the side of the glass, but also attaches itself to the fins of the fish.
Any suggestions of what they are?