First off, I'm sorry for your loss, that sucks. I'm going to tell a few stories and try to teach a few lessons. I hope this is interpreted as more educational and less poorly timed soap box speech.
I'm going to be blunt and say that there is likely some poor system design at play as well.
I've come home or walked into a friend's place to a dead or headed there fast tank three times in the last 25 or so years. The times I've seen this (in chronological order):
1. A house sitter who poured a huge amount of flakes in to the tank every day, continued to do so for the next two weeks as the fish started to die.
Did not remove any of the dead fish, and also didn't call me to say something was up. Two lessons learned. First, measure all food out in advance in weekly pill containers and, hide the rest. Second, educate them a bit about overfeeding, and leave strict instructions to call you if they have any concerns. If they do call you, even if it's about a non-issue, handle the call in a way that they are likely to still call in the future.
2. Learning CO2 on a freshwater tank. Elevated the value/got it stuck on/something like that. Gassed several, but fortunately not all, of the fish in the system. Lesson, go more gradually, have fail safes, don't run so close to the edge. Don't design systems with single points of failure.
3. Buddy declined to remove the large trash paly collection in his small tank. At some point his ATO ran low and shot a bunch of old kalk sludge into the tank. The palys got angry, did their palytoxin thing, and crashed the tank. Went from the nicest tank of its age I've ever seen to a cloudy puddle of murky sludge overnight. He also managed to palytoxin himself and ended up with visits to his GP and optometrist in short order.
First off, yes, I recommend running two return pumps when the cost of a return pump becomes acceptable insurance on your livestock. I happen to think that all drilled tanks should have a minimum of 5 holes, two for independent returns and three for drains. That said, reality often gets in the way. My current tank only has three, and that means among other things that I only have one return pump. Part of it is that it's a temporary setup, but more importantly
a failure of my return pump will not take out my system. I'm fairly certain of this because over the years I've had return pumps fail, turned them off for maintenance and forgotten to turn them back on for a couple days, etc.
There are two things that can greatly reduce this being an issue, and I'd love to hear from the OP as to if either of these were in place. Perhaps they encountered a failure I have not yet planned for, in which case leaning more about it could help us all.
1. Have a heater in the display. It might not be enough heater to keep the tank at the correct temperature in the dead of winter (ok, we don't have winter in the Bay, but you get the idea). There are actually several benefits to multiple undersized heaters vs. one large one, but it should be enough to keep the fish from getting cold enough to start killing things, or at least make the rate of cooling slow enough that you have time to notice the issue. The heater should also be in a place that stays wet in the event of a return pump failure. Horizontally at the back bottom of the tank is a good spot, but inside an overflow box is good as well. The key is to make sure the overflow box level does not fall below the level of the heater with the return pump off. A simple standpipe will prevent this.
2. Have a wavemaker in the display. Just something to keep minimal water movement. It isn't ideal, but it will cover a lot of sins.
3. Be careful where your devices go if you have a feedback loop. For example, if you have a kalk doser that is controlled off of a pH probe (a risk for various reasons), then don't put the dosing line in the display and the pH probe in the sump, or the other way around. A heater without a built in thermostat in the display and the temperature probe in the sump (or the reverse) is also to be avoided.
I'm also going to ping
@Alexx , I think we had a discussion a while about about the risks of having a heater only in a sump instead of having one behind the rocks in the back, this is a good example of what could happen.