EMERGENCY BLACK SKIRT TETRA ACTING STRANGE! PLEASE READ BELOW!?

Hi I have a black skirt tetra in my 10 gallon with platies and I did a water change yesterday so the water is very cold due to broken heater and he just lies on the bottom but breathes normally plus 3 fish died this morning from old age they were all 10 years old he is 2-3 months old probably 6-7 months in his life but please help I have him in a 1 gallon hospital tank with warmer water and he swims a bit but I am very worried please help is he going to die??