Tank Issues

[KirbyandMudkip11]

Hello! Recently, I’ve been having such a difficult time keeping my axos tanks clean (one 45 gal w one male axo and one 55 with 2 female). The axolotls themselves seem fine and their water quality is perfect (0ppm nitrate and nitrite, 0 ppm ammonia). I’ve been doing 25% water changes once a week and the debris keeps coming back after a few days. I believe the debris is mulm, and I feel like it may be caused by my fan. It’s unfortunately not allowing me to include images, but it’s been clear and brown colors. I’m not sure how to get it to stop at this point. Sorry for the low quality picture, my dad took it as I’m at college (I come home every week and do tank maintenance)
**He consumed the worm as well**
Any ideas?View attachment IMG_2505.jpeg