I'm so sorry
It's horrible when the sadness of a loss is accompanied by so much guilt as well. It's an easy mistake to make, and I'm not going to beat you up for not checking the bucket since I know you'll already be beating yourself up over that more than than anyone else ever could, and you'll be paranoid about checking buckets from now on. We've all done things that have killed fish, sadly, it's the worst part of the hobby, and it's so easy to do.
Mine was getting the wrong size filter sponge to cover the intake of my canister filter, and not replacing it when I knew it was a risk. The hole for the filter was too small, so it tore down the length of the sponge. I decided to bodge it by rubber banding it on, and putting the tiny gap in the sponge against the tank wall, planning to get a bigger sponge to replace it, but use that one for the time being so the canister was safe for when my first guppy fry were born. You can guess what happened next. My LFS didn't have the right kind of intake sponges, so I meant to measure the intake and order a new sponge, but got distracted with other things and baby fry, and didn't get around to doing it. It worked fine for a month or more, but one day I opened the canister to clean it, and found 13 fry in it
I'd put the intake back at some point or shifted it when cleaning so the gap wasn't against the tank wall properly. Many survived, somehow, but some didn't, and a couple of the ones I netting back out from the filter were clearly damaged and struggling to swim properly once back in the tank.
So much guilt. Procrastinating over a sponge that cost less than a pound killed some of my first baby fish. My dad tipped out a bucket of tankwater into the sink once that had a glowlight in it, and I found it in the sink, covered in bubbles. I panicked and rinsed the bubbles off and put it back into the tank, and it survived, but it also looked rough and never quite looked right after that. I'd place a bet that there isn't anyone in the hobby that has never made a mistake that has killed a fish. RIP Tomato, and hugs to you,
@corylover5