Poorly Betta Fish

[Aharvey93]

So. Long story but here goes I have a fighter fish called Lawrence.
He has recently moved house with me and seemed fine for the first month or so. He was in a 15 gallon tank with a few nannos. He then started eating his own tail. I thought he caught it on wood until I caught him swimming with a chunk of his tail in his mouth. I contacted the man I bought him from for advice and he said nobody really knows 100% why it happens and its not serious but keep an eye on him.
I decided to move him to his own tank because he seemed more and more itritated by his tank mates and that way I could keep his water perfect to prevent any infection in his fins. This worked for about a month. he perked up was eating being active and even started a bubble nest.
Nothing has outwardly changed care wise but all of a sudden he has started laying on the floor of the tank. He is uninterested in food and is lethargic again I am at a loss. I'm unsure if hes nipping his fins again I don't think he is.
Pics attached.

He gets water changes weekly about 20%
He gets fed a mixture of pellets and frozen betta food.

Tank
15ltr tank with filter, heater and live plants

Nitrate 20
Nitrite 0
Ammonia 0
Ph 7.1

I would really appreciate any feedback or oppinions you clever people have
Thanks

 

[Cearle]

Are you sure it’s his/her tail?

 

[Smalltownfishfriend]

Do you have any idea how old he is??

 

[Aharvey93]

Not sure on age I have have had him about a year

Are you sure it’s his/her tail?

Yeah I'm sure he was eating his own tail I saw him doing it and its about half the size it was not its a little stubby tail now. But his fins have also got a bit tattered as you can see in the pics