Carpet of white fluff?

Hi guys, can anyone tell me what this carpet of white fluff might be, or better still a treatment for it?
Its a 26 gallon tank, I only have assorted tetras in there which I have for years, I do a weekly 50% water change which is due tomorrow (and the white stuff only appeared yesterday)
I assumed it was left over food gone moldy yesterday but today ive noticed one of my tetras swimming up right and theres a chunk of the stuff on its tail
The temperature has increased this last week by 3 degrees buts its still only 26 in the actual tank

Thanks and any help would be much appreciated

Looks like mold to me. I would perform a 40% water change, along with a thorough gravel vacuum.

Thanks for your response, I do a thorough gravel vacuum with each water change, I just dont understand why its suddenly appeared after years, Is there any treatment for mold as the fish that had some stuck on its tail has since died, so would like to cleanse the full tank incase the problem repeats in the coming days or weeks tbh, cheers

My guess is that it’s just old food that has molded. Did you have any plants there at one time? It’s possible that you may have added a terrestrial plant and it died and molded.

Do you have a picture of the mold on the fish?

Ive only had mossballs in there for the last 10 months or so, but it use to be heavily planted etc, im wondering if the sudden change in temp has started some kinda bloom tbh

I didnt take a pic of the fish before he died no, his tail basically had a clump of the same white mold stuck to it though

The white ‘mold’ could have been fungus, which is a disease that is common in freshwater fish.

Have you performed a water change yet?

Here is a helpful read: https://helpusfish.com/30/how-to-remove-white-mold-in-a-fish-tank.html

Thanks pheonix and sry for the late reply, yeah I did around a 60/70% water change and vacuum this morning, ill keep my eye on it and see if it comes back within the same speed as it first appeared then look into some kind of fungus treatment for it, its just weird how it suddenly appeared after over 1.5 year considering ive changed nothing in there : /

That is interesting that it just started now. Keep us posted!