The water in the fish tank turns yellow because of the fish feces, and the residual bait accumulates at the bottom of the fish tank.
After a long time, the water in the fish tank will turn yellow, just like a basin of clear water will turn yellow when urinating in it.
You should use the sand washer to suck out the impurities at the bottom of the fish tank, then change the water properly and clean the filter sponge to improve it.