I'm so sorry for your loss. I feel that based on your description, he/she had an internal parasite and would have needed a vet's attention.
I'm glad to hear that you are using sand as a substrate as that is a common error. Also, your temp sounds okay, however, how often does it fluctuate like that? If the 4 degree difference happens too often or quickly (like when you add the ice bottle, it suddenly drops), this could stress your axolotls. How much light is on the tank? In case you didn't know, they don't like light and some plants/additional hides could help them feel more secure.
95L was really too small for 3 axolotls too so that too may have led to some stress. The general rule is about 10 US gallons or 38L per axolotl. They like floor space as they spend most of their time exploring the floor so about 12 in or 30cm is the general rule there. However, anyone will tell you, bigger is better.
Be very careful using chemicals to treat problems with axolotls. A medication that is safe for a fish may not necessarily be safe for an axolotl. If it was a fungal problem, it could have been treated with a salt bath. Melafix one of the medications that is recommended for external sores in axolotls.
A note about diet. Axolotls are freshwater and I am always confused as to whether UK prawns are saltwater or freshwater? We use the terms differently. Anyway, saltwater based food items really are not suitable for axolotls. As mentioned, the most recommended staple is earthworms. If you can't dig them in your yard or get them from a baitshop, you can order them online. I think the site that is commonly used is Wormsdirect - someone feel free to correct me. The nutritional values list Erzs is
HERE.
