I can't tell if my betta has fin/tail rot or is recovering from a fight from the petstore where I go him :(

I was in petco one day when I saw the poor little bettas. I felt bad and bought a blue and red shiny one plus a one gallon tank with a filter. I have had him for about four days and he is very active and happy. Yet I have noticed that his back fin is very short and strangly at the end. He was like this when I bought him so didn't think much of it. Then I started reading about fin rot and got worried.

I don't know if he has fin rot or if he was in a fight and is recovering. His fins dont have darker edges, his bottom fin is fine, yet his back fin is short and rigid at the edge. If I treat him for fin rot will it harm him if he is indeed recovering from a past experience? And is there a sure way to tell if he indeed has fin rot?
Answers:
Either way, add salt for freshwater aquariums. If he has fin-rot, it will cure it and if it is just damage, it will speed healing and stop it from getting infected.

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In my experiences with Bettas,when their fins look like this,they're usually on their way out (Fish Dreamland,if you know what I mean).Let him go and get a new one when he dies.

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Bettas aren't supposed to be in a tank together. They are constantly fighting each other. I had a side by side tanks with bettas and they used to get all big and try to fight each other through the tank. You need to seperate them.

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He most likely had fin rot. If he has fin rot the fins will get smaller. If he's had fin rot it should grow back. Generally most cases of fin rot are cured by fixing the original bad water condition that caused it. Unless your water is clean, and his fins are disappearing treatment is counter productive. (You don't want to build up resistant bacteria.) Keep his water clean, feed meat based betta food, and don't off feed. He should be fine.

Treatment (Mainly to make you feel better rather than your betta.)

I'd add about 1/3 of a teaspoon of table salt. (Get a cup of his tank water and pre dissolve it.) If it makes you feel better of about his condition. A treatment like mythlene blue, melafix, or primafix might prevent secondary bacterial/fungal infections without promoting resistance. (Also they come in liquid form as packeted meds are really hard to use in a one gallon tank.)

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I use salt baths on any fish that has fin or tail damage, but especially on those that have fin/tail rot. They are a good treatment for both damage and rot, that doesn't require (and is cheaper than) medicating you fish..especially if you're not sure you have to. They stop fin rots and promote regrowth if you do them regularly.

1 tablespoon aquarium salt / 1 gallon dechlorinated water at the same temperature as you fish's regular water. put fish in bath twice a day for 15 min.

a lower salt concentration of 1 tablespoon aquarium salt / 5 gallons dechlorinated water is safe to keep your betta in all the time and can also be therapeutic.