Nice setup you have there reminds me of my own. That first picture didn't do him justice. I've never seen a Betta with that brilliant of a green color. I love the pet keeper tanks. Easy to maintain and clean. I'm using them to house my boys at the moment.
I actually use a coffee mug to do my 50% water change on Wednesday nights. I use a gallon jug that was cleaned with just hot water to refill. Make sure you condition the water the you are adding. Then on Sunday nights I do a 100% change. On the 100% change I gently dump the substrate into a strainer and give it a quick rinse. Put the rocks back in the tank, add conditioner, and refill it. Add the heater back and swirl the water with it. Then add the fish back in. Also I bought a new mug and strainer that has never been washed with anything other than a bath in hot water when I first got it.
Just a thought but my daughter's betta started stressing and tail biting after she added a plant to his tank. Once we figured it out and removed the plant, he stopped. Maybe he doesn't like the plant? Is he eating? How warm is his tank? I am no expert, but maybe if we had more info, someone could help.