The air would be used for preventing a vacuum when the weight tries to expand.
By doing so, the weights can become less dense or more dense that water.
It would use a seal similar to that that is used in pumps and automotive use that goes around a shaft.
A single lip seal would be sufficient.
It will allow some water into the air chamber. One aspect of an invention is "who will buy it?".
With this, since it requires an environment like water, aquarium owners would be the market targeted.
This also would help to establish it's costs. If it can not be manufactured cheap enough,
then the market targeted will not find it worth the amount charged.
A retail cost of less than $30 should work.
With the moveable part of the weight, it's weight would need to be sufficient to over come the friction of the seal.
Even though it will rotate slowly, that would be preferred. Other wise it might harm the fish in the aquarium.
And from time to time, the aquarium owner would need to remove water from the air side.
This can be done when they are maintaining their aquarium.

An example of using an understanding of engineering and marketing.
No replies are necessry, just showing an example.
Bye