Betta Fish: How can i help my betta with a heavy tail?

this is yuri and his tank. i noticed since i got him that a lot of the time he is swimming oddly at times. sinking down tail first, even laying almost sideways with his tail curled over his body. im a new betta owner and i suspect his tail might be too heavy for him. i hate seeing him having a hard time with just being in his tank. its like his tail weighs him to whatever side its curled to.

his tank is a seven gallon with a natural setup(the substrate is a mix of sand and fluval substrate i mixed together) that i put tannins into about once a week (it doesnt look it w the light but i put some yesterday) ive had it maybe half a month now and the only tankmates he has is some bladder snails that im hoping will help me deal with the algae problem you can probably see on some of the plants.