When fish die your lake is close to terminal too
Sick lakes are characterized by two key physical changes that occur.
This means that layers that fish can live and breathe in steadily decrease so their nutrient uptake and that of the whole oxygen breathing food chain is constrained. As muck accumulates on the bottom it covers clean sandy spots where fish would normally spawn. This again means less fish. So various feedback loops kick in that all lead to less fish and less competition for invasive weeds, algae and cyanobacteria.
Virtually every common lake treatment or intervention exacerbates the problem by promoting nutrient recycling that helps invasive weeds, algae and toxic cyanobacteria compete more effectively for available nutrients so that they bloom and dominate.
Algaecides quickly kill algae so that the dead algae fall to the bottom to decompose and perpetuate the nutrient recycling.
Such as alum that clear the water and precipitate phosphorus down into the sediment to perpetuate nutrient recycling.
Conventional Aeration raises dissolved oxygen levels in the upper layers of the water column. Because phosphorus precipitates out of the water at higher dissolved oxygen levels acts in much the same way as chemical precipitants by recycling phosphorus back into the sediment.
The difference is that chemical precipitation tends to retain phosphorus in the sediment a little longer, whereas when conventional aeration fails to oxygenate the sediment phosphorus rapidly becomes dissolved in the water again, fueling weed and algae growth.
All the organisms that make up a sustainable productive food chain from zooplankton to insects, mollusks and fish need oxygen to respire. So any solution to the problem has to start with destratifying and reoxygenating the water column and oxygenating the sediment.
Once the environment is capable of supporting a productive food chain, interventions must be made to ensure that the organisms at the base of the food chain can compete and out-compete weeds, algae, and cyanobacteria for the available nutrients.