Thanks everyone.
I took Tux with me on a visit to my families place, because my vet in the city here was on vacation and because my vet in my hometown will treat rats. Around 10 pm the last night I was home (yesterday) Tux started having seizures. His body was rigid and his breathing was very light and I really thought he was dying right then and there. I didn't think he'd make it through the night.
I laid him beside me wrapped in my scarf until I accidentally fell asleep. When I woke up I put him in his little cage with a hot water bottle. The next day I was shocked to see that he was still alive, but he had deteriorated so much in the last 12 hours since his first seizure.
I called the vet and made an appointment to put Tux asleep. It was apparent that he wasn't going to live much longer anyways, and I didn't want him to suffer the last day or two he would be alive. It was so heart-breaking. It happened at 11:30 this morning and I've been crying off and on all day. He was such a sweetheart. I'm going to miss him so much.
My father buried him up by a waterfall on land our family owns, near a cabin my father is building me. I left him wrapped in my favorite scarf that he loved to crawl into, just because it didn't feel right to take him out of it.
Thanks to everyone for your support.