TLDR: My snake died, need some friendly advice on what I can do to AVOID the same mistake with the others. Please be nice, I'm new to this and still sore over the loss.

I researched (mostly Snake Discovery on youtube but I used others as well) and slowly bought the supplies to buy my first snakes over 6 months and waited until my local reptile expo to buy the snakes. I got an Albino Checkered Garter and a California Red-Sided Garter from breeders that were recommended locally by lots of people and stores. The Albino is the one that died. I got him on Oct 5, he was born June 26, died Nov 21.

The set up:

I used papertowel for substrate. I did this because I wanted to monitor the poop for a while when they were babies and I was planning on converting it to a bio-active when they got older.

Yes, I co-habitated them, as from what I understand it is ok for garters of the same age and size. I realize now, I should have kept them separate for a couple weeks in isolation to make sure no health problems arouse.

The enclosure was an exo terra glass 18"deep x 36"long x 18" tall.

Heating I used a heating pad thermostat ceramic heat bulb UVB bulb on a timer. Warm side 33 C/91 F cool side 21 C/70 F. I use a heat gun to check these temps.

One water bowl on each side, lots of plants to hide in, lots of basking spots, as well as a moisture box with sphagnum moss. I also live in a colder climate and room is in a basement so I have a space heater in the room to keep it at 21C/70F.

I fed them noodle/silver fish every 3 days with some repti calcium every other feed as per the breeders advice. I tried earth worms but they weren't interested and I was going to start introducing pinky parts soon. He was eating fine, sometimes he would eat more than other days.

The story:

Around Nov 1 when I went in his enclosure to handle him, he was basking. I picked him up and he was stiff and didn't move even after I picked him up. At first I thought he might be cold or dead but he did end up moving after a couple seconds and seemed to be fine, I didn't think much of it.

I went on vacation on Nov 7, on Nov 15 my house/pet sitter told me he didn't eat but he's also in shed. She took a picture and sent it to me but it didn't look like it was coming off in one piece, just flaking. I told her to make sure the humidity box was moist and to just leave him be. She tried to feed him again on Nov 18 and he didn't take it again. Nov 20th she said she saw him crawling around. She went in to feed him on Nov 21 and she just found him in the open, no color, and dead.

Sorry, I know this is long, but, can you see anything I did wrong? I'm home now. I took everything out, gave everything a good scrub, sanitized everything, washing it off with water, and put everything back and more decor as now the California Red-Sided is in there alone. It's probably waaaay too big for her now, so I am contemplating getting a smaller one until she gets larger. What can I do to avoid this one passing away? I'll include the pictures of the "shed" and the current enclosure.

If anyone could give me advice to keep the California Red-Sided alive that would be great. He seems fine for now and has been eating.

Thanks for the read and any advice is appreciated.

Set up

Shed