tank weight-lead donut?

[JohnAC]

On my last dive trip 2 wks ago, I noticed someones rig with what looked like a lead donut on the top of his tank just under the first stage.

Does anybody use one of these? Does it move around while diving?

I like the idea of moving some weight a little higher on the tank for better trim.

 

[fisheater]

Sounds like you saw an ankle weight being used as a trim weight. They get are usually clipped around the top of the tank, just under the valve.