Their fins are disappearing before my eyes! help!

[Indyfishy]

This morning I woke up and all my fish are missing chunks out of their fins. None of them have a history of tail-biting. I'm afraid it may be some type of fin melt or something! I noticed Daisy had a big chunk out of her anal fin last night, but didn't worry about it. Then this morning I noticed that the edges of her tail look really ragged. Then I noticed that Sully has a big chunk out of his tail, and Milton has a big chunk out of his dorsal that looks like it's been melted off. Help! What medication should I buy, I have AQ salt already! I did a 100% water change on all of them last night!


Housing
What size is your tank? all are 2.5 gallons
What temperature is your tank? 75-80 degrees consistently
Does your tank have a filter? no
Does your tank have an air stone or other type of aeration? no
Is your tank heated? not at the moment, we've had a major heatwave and the heaters were making the water too warm.
What tank mates does your betta fish live with? none

Food
What type of food do you feed your betta fish? Omega One Betta Buffet Flakes, Top Fin betta pellets and the occasional freeze dried bloodworm
How often do you feed your betta fish? once a day in the evening, but Milton gets fed more often because my mom feeds him.

Maintenance
How often do you perform a water change? twice weekly, one 50% and one 100%
What percentage of the water do you change when you perform a water change? see agove
What type of additives do you add to the water when you perform a water change? stresscoat, and that's it.

Water Parameters:
Have you tested your water? If so, what are the following parameters?
N/A
Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? All of them are losing finnage! huge chunks missing, Milton appears to have a spot that looks melted!
How has your betta fish's behavior changed? No behavior changes thus far that I can tell!
When did you start noticing the symptoms? Last night on Daisy, this morning for the other two!
Have you started treating your fish? If so, how? Not yet, leaving for Petsmart now, will check this on my phone for any feedback!
Does your fish have any history of being ill?
How old is your fish (approximately)? Daisy is almost two, Sully and Milton are nearly 1 1/2.

[bettasareawesome]

I'm almost certain thats fish rot. one medicine I've heard about is melafix I think its about $9 and is probably at local petshop or walmart.

[ZackyBear]

How many fish do you have in the 2.5? It sounds like you have them in the same tank...

If they are, then they're attacking each other. you need to separate them immediately. Warm and clean water will help them heal their fins. Though I may just be reading wrong.

I would't trust anything with -fix at the end of it. Very risky and can cause fin melt, though some swear it's a miracle worker. Can we have a few pictures?

[Indyfishy]

Oh no, they're definitely not in a tank together! Sorry, I should have specified, I was in such a rush! I've put aquarium salt in their tanks for now, and if they've worsened any by tonight then I will be adding tetracycline to their treatment plan. It's definitely not fin rot, no red or black edges on any of the wounds. Melafix is not meant for bettas, it burns their labrynth organ (it has tea tree oil in it, which is an irritant).