I have to say that it is highly unlikely that not switching the airstone on was the primary cause of your losses, in modern aquaria with power filters air stones are completely unessesary except for in extreem circumstances; ie. during a heatwave when high temperatures reduce oxygen capacity or when medicating. None of my systems have airstones and all contain large/predatory fish many of which have high oxygen demands.
Unless there is a more obvious cause such as forgetting to add dechlorinator the most likely cause is that something in the tank is using up the available oxygen and depriving the fish, rotting food trapped in the substrate or filter is the most likely cause, juwel filters are infamous for trapping decomposing gunk in the bi pass chamber inside the filter housing, stick a syphon down inside the chamber and see how much crap comes out.
Your decision to remove the juwel filter could well be the best you have ever made, i have long been preaching about the inadequacies of these filters for anything above planted community tanks with small fish.
i didnt think airstones made a lot of difference
