Another vote that low oxygen ...

Another vote that low oxygen is BOD, COD, DO related, another angle is that either a new group of bacteria have gotten in somewhere, or as is happening now, check for glyphosate, the main ingredient of Roundup now found commonly in foods, it has a detrimental effect on beneficial bacteria and can tip the balance on bacterial populations quite easily, very potent and for example is preferential to Salmonella.