Howdy!
First off, let me say I'm super impressed at the time and care your family has taken to make sure your fish are looked after. That's awesome!
Looking at your beautiful fish, I don't immediately have an illness that comes to mind, though I'm only a year into fishkeeping myself. I'd echo what others are saying with a few added possibilities/solutions:
1. Clamped fins are often an indicator of stress caused by an illness, not an illness itself. Sometimes it can even be just stress. The easiest solution could be that Vox is getting chased or bullied and is stressed/sick because of that. Are there any other males in the tank that are chasing, nipping, or sparring with Vox? I've never had guppies but from my time owning platys I know the males can fight over EVERYTHING.
2. What's your water quality like, and do you use a dechlorinator? Tap water has chlorine/other metals in it if you're using your cities water supply. It's okay to use for fishtanks, usually, but you should dechlorinate your water all the same. I like the one API sells, personally. If you're on well water, like I am, treating fresh tank water for heavy metals is still recommended. Follow the dosing instructions and add after each water change.
3. How long have you cycled your tank for? There are great articles on tank cycling that explain this better, but essentially "cycling" a tank involves kickstarting the ammonia in your tank, allowing good bacteria to grow and basically beat back the nitrate/ammonia/nitrite. Along the same line of thought, while cleaning your tank and doing a 50% water change is the correct thing to do, I have found cleaning too much too soon can rid the tank of good bacteria build up. How often do you change the water? Do you clean the filter as well? What's in your filter media?
4. At the end of the day, sometimes fish get sick for no apparent reason. If you bought Vox at a chain fish store, it's not uncommon for fish from there to be stressed, have low immune systems, and illness. If nothing works and Vox passes away or has to be put down, you and your daughter should be proud of how well you are taking care of him and his buddies. 🙂
-I also have only one tank. I use a designated medium sized glass tupperware bowl for dosing and observation.
-Just spitballing, I would look into methylene blue. I don't know exactly what's going on with your little friend, but I have used methylene blue myself for a fish with cotton wool disease and it worked great. Some people swear by it, others have mixed results. Methylene blue has been used for everything from dropsy to fungus to mouth rot. It is SUPER stainy though, so maybe help your daughter with it. Salt baths are also great, just make sure you dose correctly.
Good luck! I hope Vox feels better soon!