all that I have in there is live plants.. driftwood with anubias (i think thats how you spell it) and there are 2 other plants but I cannot remember theyre names. Im pretty sure one is a banana lilly and the other not sure
most likely suspect is the driftwood releasing tannins into the water making it yellow. You could chuck in a carbon filter, they help get rid of discolouration in the water.
Bettas love tannin stained water so you dont need to worry about it harming your fish, although it doesnt look nice to us...
The wood will continue to leach tannins for a very long time no matter how many water changes you do
I got lost watching Supernatural lol I would agree, the driftwood. I added some cactus wood to my main tank and its still yellow in there. Harmless. Now you can sit back and breathe lol Good thing you asked!