It's oxygen depletion kasi they breath it at the same time gumagamit din ng O2 ang decaying matter. Ammonia takes time to build up lalo na kung may filtration ka na tapos nag siphon ka pa ng feces and uneaten feeds. Mauuna na sila mamatay sa asphyxiation kaysa sa poisoning.

Btw, re your question kung maselan ung isda sa taas ay tingin ko hindi kasi these are tilapia, genetically engineered siguro, even flowerhorn is tilapia a d tilapia is a hardy species. Ung arowana yata maselan, mataas ang dissolved oxygen requirement nya either as a species or by it's big size (biomass) relative to the aquarium.