My personal rule is that I'll never buy a sick betta fish from a store unless they give me a discount on the fish. I'll also never buy a fish that I know will die.
This sort of comes from where I used to work, when I was keeper of the betta.
Basically, stores understand money more than they understand morality. When you're buying something at full price, you're telling the store, "Yes, I like what you're doing, keep doing this," and telling them that sick fish will still sell. When you ask for a discount you're telling the store that the fish is not worth as much when it is sick. Stores do not want product that can only be sold at a discount
Buying a fish that you know is going to die is removing the responsibility of looking at the dead fish and disposing of the body from the store. Knowing that a fish died and looking at a dead fish are two different things.
This means that if a store won't give me a discount on a sick fish, I have to walk away. I have to leave that fish. It is not easy. It's never easy. But it has to be done, to teach the stores that their fish need to be taken care of.
I'm trying to find the video of Steve Irwin, the Crocodile Hunter, talking about buying illegal animals at a market. He's filming this video at a market in some country where people have taken wild animals, put them in tiny cages, and are selling them as novelties or pets or for their fur (I don't remember why). You can watch Steve's heart break as he's explaining that he has room for them at his zoo and he has enough money to buy all of them and give them a home, but for every animal he buys, another one is caught and put in its place in the market. I know that Steve Irwin loved animals more than I am capable of loving anything, and if he could walk away from those animals, I can walk away from a betta at Petco.
This doesn't mean that you should only buy betta fish in perfect health. A little fin rot or a little skinniness is normal from a fish that's been living in 1/18th of a gallon for more than a couple weeks. It does mean, however, that you should consider the quality of life of all the fish in the store before you buy an extremely sick fish.
I'm going to call this Cup Sickness. A lot of fish who stay on the shelves at the store, or who live in enclosures too small (LOOKIN' AT YOU, BETTA CUBES, FUCK YOU, BETTA CUBES) get this combination of problems.
~They lose the ability to control how they float. This usually means they're stuck on their sides at the surface of the water, but sometimes means they can't float and are stuck at the floor of their cups. Losing their ability to control their floating is called swim bladder disease, or SBD
~They become either extremely skinny or seriously overweight (or both, which, yes, is possible). A betta's weight is easiest to see when viewed directly from above.
~They often have very poor muscle tone, from not being able to swim in a straight line
~They are often constipated, and may have a swollen tummy.
IS NOT CUP SICKNESS
This is called Mystery Bloat or Malwai Bloat. This fish was not having trouble floating or sinking. He's severely swollen in all directions, like he swallowed a marble. His scales are sticking up. This is not cup sickness. Mystery/Malwai bloat is contagious and fatal for betta fish. Do not bring a fish with this sickness into your house.
Back to Cup Sickness
There's four main things that can cause these symptoms. At least three of these causes are usually present in cases of cup sickness.
1) High Ammonia Levels: Betta fish have a very high tolerance for ammonia. They can stay alive in cups with 32ppm of ammonia or higher in them. 1ppm of ammonia is considered the lethal level for ornamental fish, because living in 1ppm of ammonia for 4 days kills 50% of fish. Betta fish can stay alive in higher levels of ammonia, but they do start getting physical damage from it. Living in 4ppm of ammonia is just living in and breathing poison.
2) Small, Cold Living Space: If their bowl is a size of shape where they can't even swim their body length in a straight line, they are going to develop weird muscles. If there's no stimulation in their bowl, they're going to get bored and sit on the bottom. Cold water also makes them sleepy, contributing to cold. Water that is too cold also makes it harder for their digestive system to work properly. The combination of these things means that many fish get seriously overweight, and if they stay in the cup for longer they start losing all their muscles. They become extremely thin and very small.
3) Awful Food: Most betta foods on the market are just kind of crap. Betta fish cannot digest plant matter at all, but a lot of betta foods have grains in them as binders or fillers. Constantly needing to crap out over half the food they eat, combined with the fact that they can't move enough (moving aids in digestion) is just a recipe for serious constipation.
4) Damaged Slime Coat: Betta fish have a mucus-like coating on them, which their bodies make to protect them from physical damage and illnesses. Their slime coat can be damaged if they're sick, but it can also be damaged when they move or are touched too roughly. A lot of store carrying betta in cups don't have the time or skill to make sure the fish are not stressed or roughly handled every time the water in the cups is changed. Children of shoppers like to pick up fish cups and shake them (AND IF I CATCH YOUR KIDS DOING THAT I WILL GIVE YOU A PIECE OF MY MIND) This means that even if the cups are changed two or three times a week to keep the ammonia levels lower, the fish are still being damaged.
Usually, Cup Sickness is caused by 1, 2, and 3, with an occasional appearance by 4. So, your fish has cup sickness, or you bought a fish with cup sickness, or your friend was going to get rid of their fish because it started swimming sideways...whatever it is, you have a sick fish on your hands.
A lot of people are going to tell you that you need to feed a pea to the fish and they'll be fine, bathe them in Epsom salts, blah, blah.
Adding aquarium salt to the water of a bloated fish will make the bloating worse. Throwing that out there.
So, let's go over the basics of treating a sick fish really fast:
First of all, betta fish are small and the wrong treatment can take a very serious toll on their bodies. You don't want to give a fish antibiotics if you're not 100% sure they need them, because they can stress the poor fish's kidneys. A bath in Epsom salts can also stress out their kidneys and their respiratory system, which will already be damaged if they are living in a high-ammonia environment. You don't want to put anything in their digestive tract that can damage it unless you know it is necessary. Unless you are very sure than the betta's condition is rapidly deteriorating, you want to use the smallest and least damaging treatments possible, working your way up until you find the most gentle treatment that works.
So, the first thing that causes cup sickness is ammonia levels, and the first treatment is to get them out of high ammonia water. This means getting them into a larger tank or tub.
Unless you have a filter already cycled* that has a very low output, I don't filter hospital tanks for cup sickness. When the fish is trapped at the surface, small bubbles will move them constantly.
Getting them into ammonia-free water is the first step. The next step is to make sure that their water is a good temperature. Get a heater that you can control the heat on, and that has an internal thermostat. Set it to the higher range of their acceptable water levels (I like 80).
Instead of putting 80 degree water in the tank, I try to match the temperature of the cup the fish is in. I then add the fish before I turn the heater on. This changes the temperature of their water much more slowly.
So, they're in ammonia-free water that is heated, and at least three times as long as the fish's body so they can swim. What now?
Now you just wait. Unless you /know/ the fish has not eaten in the past week, don't feed him. Don't put peas in there, don't bathe him or add salt or anything. Put him in good, clean water, heat it to the right temperature, and let him be for a day. Keep checking on him, make sure he's not panting or deteriorating, but just give him space and clean water and give that a chance to do its thing.
In some more mild cases, enough water is all you need.
This is Cinco. Cinco had been in a cup at work, stuck on the bottom and unable to reach the surface without fighting, for five months. Where his swim bladder was, the side of his body had caved in. I thought he had a birth defect and that he would never be able to swim.
Brought him home and put him in a bowl I'd carefully built to accommodate a betta who can't float. Went to sleep, woke up, and he could swim fine. He's still very sensitive to water currents and ammonia levels, but for the majority of the time he swims like a normal fish.
Sometimes, though, water isn't all you need.
Once the fish has been in his new water for about twelve hours, it's time to feed him. Make sure that what you're feeding is good quality food. He's probably got all sorts of weird shit in his digestive tract from eating whatever Hikari shit the store's feeding him. I like to limit it to one or two pebbles (I feed Omega One, which has larger pebbles) and no more. Then it's a matter of waiting and counting if he poops.
This is why it's a good idea to not have gravel.
If it's been 24 hours and your fishy hasn't pooped, it's time to feed him something with fiber. A lot of people suggest a pea. Peas are entirely plant-based, so the betta cannot digest any of it. In theory, when he shits that fucker out, it'll take out anything else in his digestive system that might be blocking it.
To feed him a pea, take a frozen pea and microwave it for about 10 seconds. Then use your fingernail to pull off a section about the same size as a food pebble, and drop it in the tank. If your fish is aggressive, desperate, or kind of dumb, he might go right for it.
If you just can't get him to eat a pea (can you blame him?) go to the pet store and get frozen daphnia or frozen mysis shrimp.
Both of these are things a betta might naturally want to eat. Daphnia have a natural laxative effect and mysis shrimp have an exoskeleton that's all fiber. They're also much easier to feed than tricking him into eating a pea.
If you're on day three and the swelling just will not go down, then you can try to give him an epsom salt bath. I pull half a gallon of their tank water out into a pitcher, add two teaspoons of epsom salt and dissolve it. Then I transfer the fish into the pitcher and watch him like mad for ten minutes. If he starts panting or panicking, I immediately transfer him back. If he stays calm for all ten minutes, I transfer him back to his tank, and then add half a gallon of fresh water into his tank to replace what I pulled out of the tank.
Honestly, this has never failed me. In some cases it's taken weeks, weeks on constant ammonia checks and water changes and baths and poop counting, but all of my cup sickness rescues are now better and normal fish.
And it's worth it.
*We haven't talked about cycling yet, but we will