Let's not jump to conclusions here. It's completely possible that the thread is actually just slime coat coming off as it often comes off in grey-ish sort of clear strings from anywhere on the body. Fill this form out and if you can get any pictures, please post any and all of them. Best time to get photos is when the fish are eating.
Housing
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
Food
What type of food do you feed your betta fish?
How often do you feed your betta fish?
Maintenance
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Water Parameters:
Have you tested your water? If so, what are the following parameters?
Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
30 gallon / 112litres
Uhm atm 30 celsius!
Yes aquaels filter!
No airstones. But I use the filter to stimulate the surface for my oxygen.
Heated, Yes.
4 guppys and 2 siamesis.
I feed them with white/black mosquito larva and artemia.
Atm I feed them every third day beacuse of the medication and I want the water to stay as clean as possible. Before it was once everyday.
WC once a week, about 30%. But this/ last week twice one 50% and one 70%.
I use AquaSafe before adding the water to eliminate chlorine.
Ammonia: 0
Nitrite: 0
Nitrate: 10
pH: 6.8
Didnt test the other two.
They all started scratching themselves a little, for about 2 weeks ago. Wasnt so bad they all seemed happy so I did a waterchange beacusr i thought it might be bad water. Later on checked the water and nothing was wrong. This week they have all began looking so sas swimming with their fins next to the body, they can swim normally so its not fin rot they spread them from time to time. One have become very pale but it seems to come and go.. My other fish scratch but their appearance havnt changed (guppy and siamesis). Today one of the females tried to burrow her head brutally in the gravel

scared me a lot. Also as you are saying the threads does not look like worms! They are barely visable and comes out of the whole body now so i would take a guess its the slime coat. My male betta who shared the same water before with a divider has the same symptoms with the slime coat. I will try and take some pictures.. :/ they are all pretty new. Oldest 1month newest around 2 weeks.. when it all started. No history of being ill i guess since they are new