Unfortunately, I have sad news. Squirt died.
I have absolutely no idea what caused his death. He was swimming around and happily blowing bubble nests just yesterday. I checked the thermometer everyday so the water temperature was fine, and I tested the water yesterday and the water was fine too.
He did have some somewhat clamped fins (they were stuck together on the ends) which indicates stress or disease, but he had no other symptoms so he didn't have a disease, and by itself clamped fins cannot be treated. He was very quiet the second day but yesterday he seemed very happy and perky and he had no lack of appetite definitely! He kept chasing my finger around to see if I had more bloodworms (those were definitely his favorite)
He blew a TON of bubble nests over the few days I had him so I at least know he was happy. (betta fish blow bubble nests only when they are happy and healthy and ready to breed).
He may just not have been able to cope with the stress of moving to a new tank. Or it could be that he was just not very healthy from having to live in a pet store cup situation. He may have been old too.
Since he didn't show any signs of diseases, the tank should still be fine, I am going to do a big water change and get some better bacteria for the tank tomorrow, and then in a few days I will try again with Squirt the second.
I thought that maybe it could be because that the tank is not fully cycled yet, but the ammonia is still right around zero so it couldn't have been that, but I am still getting some better bacteria/bacteria supplement for the tank just in case.
I will miss Squirt.