Long String of White at molly’s tail, fin rot?

[Rollxr]

So i noticed today that my molly has a long strand of white at the back of her tail. And before she never had this. Is this fin rot? Should i start adding in seachem paraguard? My water parameters are.
general hardness: 214 ppm
pH: 8.0
Ammonia: 0
nitrite:0
Nitrate zero cuz i just water changed recently.
Could the cause possibly be i was feeding bad foods? Sometimes the flake container would be open and i would give it to her which is my fault.
The tankmates are 1 other female platy and this molly in the photo is also a female. I have a sponge filter and a internal filter.

 

[Colin_T]

Excess mucous produced by the fish because something in the water is irritating it.

Test the water for ammonia, nitrite, nitrate and pH.

Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.

Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.

Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.

 

[Rollxr]

Excess mucous produced by the fish because something in the water is irritating it.

Test the water for ammonia, nitrite, nitrate and pH.

Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.

Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.

Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.

It isn’t mucus, its a strand of white attached to the molly’s tail as if it looks like the tail is melting? I have provided my parameters from before as i said in the description……
Amonnia; 0
Nitrite: 0
Nitrate: 0
general hardness: 201 ppm
pH: 8.0

 

[Colin_T]

The only fish disease that is white and long is anchorworm and that isn't anchorworm.

If the fish produces excess mucous it can appear as a long filament or filaments coming off the tail. Do a big (75%) water change and see if it helps.