Looking for feedback on my tank

Hi all, I recently purchased my first axolotl. He was a juvenile leucistic, and was around two inches. He passed away about a week after we got him and my fiance and I were pretty devastated as we have been wanting an axolotl for a long time. I'm not sure if it was something we did wrong or not but I wanted to ask for a little feedback on my tank and any recommendations for any changes you think would be a good idea. I want to make sure we have everything as optimal as we can before we purchase any more babies. We have a twenty gallon with sand as substrate (we are going to switch to slate in the next few days tho). We had a spray bar for our filter but it broke so now we have it pressed very close to the glass. We use a fan to cool the tank and it stays around 64-66 Fahrenheit. I do 25-30% water changes once a week, and the tank parameters seem alright but not quite perfect. No ammonia and the pH is around 7.6, but the nitrites tend to be around .25ppm, and the nitrates 5ppm. Every time I do a water change I add a nitrifying bacteria but my fiance thinks we don't need to be right now. In the tank we have two ghost shrimp and two mystery snails to help keep the tank clean. I'm not sure if having them could cause any problems, the snails are pretty large so I don't think the axie would be able to eat them? We have a small booklight that we are using as a light source. With my previous axie I kept it off pretty much all the time, but I've heard they actually need a light cycle so I think I was doing that wrong. I know this is a lot of info but I just want to make sure I get all of the best feedback possible, I'll put some pictures of my set up below. Any responses are appreciated!

It you are getting readings of nitrite or ammonia, your tank is not yet fully cycled. Personally I don't use or recommend the bottled bacteria as I feel it gives a false sense of security. But it is really up to you whether you do or not. But I would make sure your tank is fully cycled before adding your new axolotl.

Snails are a heavily debated and sensitive topic. You would be surprised what axolotls can fit in their mouth. My personal preference is to just not have snails in my tank. I feel very small snails are fine but ones that can fit in the axolotls mouth but are quite large may pose an issue. So I would mainly be ok with either very small snails or large snails that cannot fit in their mouth (must be larger than the axolotls head). However I have also heard bad things about snails with trap doors but not sure how much of an issue these are. I prefer to err on the side of caution and just not have them in my tank to be safe.. The other consideration is the living conditions for the snail as well as the axolotl. I think it is important to ensure you can easily accommodate both species within their optimum living conditions (eg temperature, PH etc). My understanding is mystery snails are tropical snails and prefer warmer water. So although they may do ok in the colder water, they won't thrive.

It is up to you whether you want lighting for your tank. I have no lighting at all. What you have should be fine. Lighting is ok, as long as it is not too bright or the axolotls have nowhere to escape the light. You have a nice hide setup .

I can't see any main issues with your setup but some more live plants will add a bit of colour and make the tank look more interesting

Thank you for your comment. I knew snails were a bit of an issue here but I guess I impulsively got them. I had no idea they were tropical tho. I have a friend with a tropical tank, so I will be rehoming them with her.