I think that your general model is wrong. Briefly, here’s a couple of reasons why:

First, producers strongest economic incentive is net-profit maximization.

Net-profit= #fish sold * (average revenue per fish sold - average cost to farmer per fish sold)

Farming fish at quite high stocking densities without counteracting aeration causes low dissolved oxygen levels. These high stocking densities cause a greater number of fish to be sold. As long as the increase in net-profit caused by the increase in the # fish sold is greater than the decrease in net-profit caused by the decrease in marginal profit per fish, farmers have economic incentive to do that. Therefore farmers would have their strongest economic incentive be in favor of some negative outcomes caused by lower oxygen levels as long as these were outwieghed and lead to them increasing their net-profit, namely through them being able to farm a greater number of fish.

Second, fish farming is still a young and rapidly evolving industry so farming practices may not totally align with economic incentives. One sentiment I have heard expressed is that for fish welfare there still are a lot of either welfare and economic wins-wins or win-ties. That is, there are opportunities for fish welfare to be improved without costing economic productivity.

Third, regardless of the incentives or possibility of welfare and economic win-wins or win-ties, it still empirically seems true that a) many farmed fish seem subject to sub-optimal dissolved oxygen levels and b) mass die-offs are not rare. Given the frequency of sub-optimal dissolved oxygen and mass die-offs this is probably evidence farmers currently don’t overall have very strong incentives to prevent these issues. So, while there is limited evidence in general on the topic, here are some of the counterexamples to the general model you seem to propose:

In channel catfish, the most farmed fish in the US, “[t]he traditional pond system typically produces 4,500–5,500 kg/ha of catfish with a maximum of 7,000 kg/ha (Brune, 1991; USDA, 2006). However, today, many farms in Alabama produce more than 10,000 kg/ha, and the amount of aeration provided is not adequate to consistently maintain minimum dissolved oxygen (DO) concentrations above 3 mg/L (Boyd and Hanson, 2010).” (Brown (2011), p. 72)

E.g, “For catfish, one of the major causes of significant die-offs is low oxygen, while oxygen levels are something that can be feasibly controlled.”

E.g., “Caged salmon companies have reported over 760 mass deaths to the Scottish Government in the last three years.” This article reports that oxygen shortages are a frequent killer.

E.g., The report linked in the OP estimates pre-slaughter mortality rates of 15%-80% for commonly farmed fish over the entire production cycle.