I JUSt changed my tank water. I try and keep it just warmer than room temp because I have a divided half gallon tank with no heater, I just use this really hot lamp to warm up the water. I changed the water and put both of my fish back in. Rainbow is just fine totally normal. After I put the tank back in my room I washed my hands and came back and Cosmo was floating with his head down. No clamped fins, no breathing. I freaked out and put him in a small container. When i touched him he would slightly struggle. I ended up putting him back in the tank because the small container was cooling off.
I use tap water with betta water conditioner. I have no way to test ammonia or ph or get to the store within the next 24 hours.
It sounds as if he is in temp shock. Did you float them and acclimate them to the chemistry prior to putting them in? Without a thermometer to check the temps, and acclimating them properly they can easily go into shock.
I would QT him in his own container.. usually a one gallon is recommended..
But since you have 2 fish in a half gallon.. I would use a tupperware container that has no soap residue. I would lower the water to 3-4 inches to help the ease of breathing.
With such a small container, using a hot lamp is not a good idea. You want to avoid temp fluxuation, which is what is happening when you turn the lamp on and off. Bettas are tropical fish and they need temps of 78-82*F. As well as you should be doing every other day at least 100% water changes with 2 fish sharing that tiny of a place. The build up of ammonia and the nits is going to be fast and harsh. How long have you had them?
Normally I don't get on people for the sizes of their tanks. As long as they are properly cared for. I wouldn't mind as much if you just had one in a half gallon as long as you do the water changes every other day that is needed. But two sharing a half gallon is a very bad idea.
They need space to actually swim and turn around. They need medium to heavy planting (fake or real), they also do well with a place that has hiding spots such as a cave.
You have two enemies who will fight to the death living so very close together, with no place to hide and get away from the other. They will be under constant stress from their proximity to one another, the small living space, and the very often water changes that is necessary. I'm sorry, but that sort of living condition can and will easily lower their life expectancy by a few years or more.
I know they sell these containers as "homes" but most just use them as QT tanks for sick fish rather then for permanent homes. If it's funding that is low, can easily get a 2 gallon kritter keeper at Petco for $8 and that will be plenty to thrive in.
I'm sorry if I sound rude, I'm just frustrated that companies sell those containers and claim it is fine for the fish. It's really not. When you have a healthy betta that is happy in his enviroment and thriving you will see just how each have their own personalities and love to interact with people. I don't blame you for trusting what the box says.. but all I ask is to trust us here to help you to learn. Just think to yourself "Would I put any other animal in such a small container that they could barely turn around? Would I want to be in there?" They may be "just fish" but "just fish" do feel pain and stress, and they do know when they aren't happy and healthy, and they get to recognize their owners and in fact, you can teach them tricks even lol..