Betta Tail Biting?

Hey guys,
I need help with my betta. I'm pretty sure he has been biting his tail. I understand his tank could use some more toys/decorations to keep him busy, but I'm more concerned that it is the size that is boring him into tail biting. He never did this when he was still in a 10 gallon.
I don't know what to do. Maybe it isn't tail biting at all. If it isn't, could the fake plants be causing it? I want live plants but I can't find any anywhere and I'm not sure I'm willing to spend $60 in shipping for a few live plants.
The water parameters are 0, 0, 5ish.
I have attached pictures below. The lights are off right now in case you're wondering.

 

Yes that is fin nipping. Bettas will nip their own fins from boredom or stress (many factors). Bare tanks, bright lighting, vibrations and etc can also cause stress then leading to fin nipping. I would try to add some live plants . You'll be amazed how good a 10gal can look fully planted. I also suggest adding him some Tannin Tea & other botanical extract to help promote fin growth back and prevent any infections.
TanninAquatics is perfect for betta botanicals.

You also need to be doing daily WCs to prevent any fin rot, as fin nipping can turn into fin rot very easily. Make sure you tint his water heavily with Tannin and provide extra clean water.

 

Along with what BlueberryBetta said I think it can also be hunger and aggression, that’s why bettas fin nip a lot in dark black containers. What size tank is your betta in right now?

 

The good news is once you get him to stop biting the tail will grow back very slowly but surely and eventually colour up. New growth will look like Saran Wrap
My betta stopped tail biting once he got a tank upgrade, he was bored in the smaller tank.

 

Along with what BlueberryBetta said I think it can also be hunger and aggression, that’s why bettas fin nip a lot in dark black containers. What size tank is your betta in right now?

6.5 gallons.