I am glad you were able to upgrade your Sushi to a bigger tank
The fact that you don't test your water regularly, could I assume that your tank's cycle is not established? If you don't cycle your tank, then you need to change your water more frequently which may not be ideal for you, so you may want to consider cycling it? Otherwise, for uncycled tanks of that size, you probably need to do 100% water change weekly. I hope someone else who is experienced in this can give you better advice. Try PM OFL, Sakura, Myates etc., they are very experienced and helpful.
My experience is that crowntails are more vulnerable to water quality issue. So you have to make sure the ammonia/nitrites/nitrates levels are all within normal limits. It is ideal if you have your own test kit as well. Thermometer is very important as the heater's built-in thermometer may not be 100% accurately calibrated to the surrounding, so having your own will help to confirm what you have set is what is giving out.
If I were you, I would put him in a small container for aquarium salt treatment and float this container in his tank to maintain the same temp. 1 tsp of aquarium salt per gallon. So basically pre-mix 1 tsp of salt in a 1 gallon clean jug, add your conditioner, then pour into the container. Leave your Sushi in the container and change the water completely every 24 hours for the next 10 days. During this time, continue to monitor for worsening or improving of his fin rot as the treatment plan may need to be modified.
Oh...you said feed him once a day, how much do you feed him?