Will it improve fish and other marine life stocks.
After all we pump air into fish tanks to keep fish healthy. So why not the oceans?
We do use aerators in fisheries. Dunno how much good it’d do in the ocean depths–those are pretty big.
Because the ocean is absolutely huge, and because it’s only necessary because the fish tank is absolutely packed with fish, compared to the ocean.
Also, to keep that super high density of fish alive we have to sprinkle food into the tank every day, as well as constant oxygen. That would be trillions and trillions of tons of fish food required to make the ocean like a fish tank.
No it wouldn’t make any difference at all; dissolved oxygen levels are not a limiting factor in most natural fish populations. It is important in artificial environments
like aquaculture tanks where stocking densities are comparatively super-high. Fish farm tanks (which aquariums are miniature versions of) are similar to feed lots where you cram way more cattle in than a pasture could naturally sustain. When you do that you have to add extra food and remove extra waste.
Probably the biggest thing we could do to improve weakened fish populations is to reduce or stop harvesting fish.
Maybe someone tried it, and that’s what made the Bloop. 
In general, anything we do on a sufficiently vast scale to affect the environment might have significant unforeseen consequences.
You’ve heard the old saying, “don’t mess with what you don’t understand?” Well, we have a long way to go before we understand enough of the interactions between environment and the life forms that populate it to predict changes with confidence – and in the case of the deep ocean, we know very little about the life forms that live there-- let alone how they interact.
This is the kind of thing Jeff Goldblum would warn us about.
And the species that currently live in the depths of the ocean are adapted to the relatively low oxygen levels. Increasing the oxygen level might well kill them off.