‘synirr’ found an answer: build his fish a custom buoyancy device to help it maintain its position in the water column! Based on what is seen in the videos, it appears he build the sling out of soft silicone tubing and cradled the fish in the center of the loops. While it’s hard to see, a black string is seen at times attached to the top of the sling extending upward. While not shown, I would surmise it probably attaches to some sort of floatation device at the surface.
According to synirr, who made the below two videos: “[This is] my disabled goldfish scooting around in her newly redesigned sling. She has trouble maintaining buoyancy on her own. She looks a little silly, but it is better than lying at the bottom of the tank all day!”
In his first video, he shows the fish lazily tooling around the tank in its custom sling. In the second video, synirr hand-feeds his crippled goldfish. That is dedication!