Hi.
So there are a couple possible issues which you might be interested in.
Parasites can cause bent spine / hollowed tummy area, bad breeding (or fish which should have been culled and not sold by the breeder.) vitamin deficiency also can cause the spine issue.
To treat parasites, I personally prefer the flake food option and liquid in the tank as medication. Soaking the medicine with the fishes food seems to be rejected by a fish which usually really isn't that interested in eating anyway.
Aquarium Solutions PraziPro has high reviews. Have not needed to use this and I usually use the flakes on new arrivals. active ingredient praziquantel
Usually I give my fish this medicated flake if needed. Fenbendozole. Dewormer I
https://angelsplus.com/collections/fish-medications/products/medicated-deworming-flakes
You can try to get this food from a lfs, immediately. If they do not stock it for sale, they may have their personal medicated food stock and you can ask if they would sell some of it to you. Otherwise online will get there in 2-3 days usually.
Try adding a drip of garlic juice to the food in the mean time for the fish, just squeeze a garlic clove onto the food before feeding.
I would add Epsom salt to the fishes water. Plain Epsom salt (ES), no oils or additives, can get at grocery store usually or pharmacy.
The way I treat with it is take a couple cups of hot water to dissolve the ES, when nearly all combined, then add a cup or 2 of cold water whatever is needed to be the temp for the fish's water....then add correct amount of dechlorinator /always use it with tap water (even if I forget to mention it).
Adjust the quantity of water to mix it for your tank size.
The amount of ES I would recommend is a slightly rounded tablespoon per gallon of water.
Since you feel the fish is really not doing well, adding the ES mixture a bit more quickly than usual, would be my recommendation.
You can add the ES mixture to the tank / container, remove the equal amount of water you will need to add.
Normally I would add it over 24 hour period.
But in this case, I would say every hour, add a little of the mixture, maybe about 6 portions. Then you can make up this mixture ahead of time for the complete water change, 100% every day.
Giving him the best possible water will help him to heal.
When the Bettas (or any fish) are shipped, and also held in the little containers the water gets toxic pretty fast and can harm the fish of course and cause them to become compromised - any possible issue would be magnified.
***Are there any other problems other than not eating and curved spine and hollowed stomach?
