My info is based on observations and what I can rule out or at least, seems not very likely.
If it were environmental, why do the few that remain live through the 9 levels of Hell?
I cannot rationally accept that.
I've rarely ever seen them eat any food I've offered.
Tanks where there are more algae eaters= less otto's.
I've added and resupplied more later, only to revert back to the standard 6-8.
Why are we left with some and why do they do very well for years, while the initial die off is rapid and then tapers slowly?
Food is the only decent hypothesis I can account for.
Similar observation based things for algae, plant growth, breeding, shrimp(rampant myths occurred really fast as that hobby took off), Fish health etc........help address the myths and hopefully narrow the potential causes down to a few that you can look at more closely and if lucky, rule out another 1-2 more.
But this takes time and experience, something many on line lack.
I really do not know much, but I know what I can rule out.