Other than quality water the one thing most fish need is structure, otherwise known as habitat. Submerged x-mas trees are part of that solution.
I also like what they did at Quincy a few years ago. The water was down about 15 feet or so and they went around and placed boxes of old pallets nailed together on the dry bottom. Lots of structure built with basically waste material.
Unfortunately they placing of new structure is limited in existing bodies of water. But when the water is low it would be a great time to create structure using whatever material is available, rockpiles, xmas trees, concrete slab piles from demolished building roads etc. The sources are infinite.
An overnight quality improvement would not be seen, it would take a few seasons to see things really improve, but I could wait.