Yiur tank is alady over stocked. For a betta to be able to be in the same ank as any species of cories and for both soecies to thrive happily you would need a tank minimum of 10 gallons. Even for the dwarf species, they are extremely active and need a large footprint to swim around in and thrive. Corydoras are also schooling fish, they need to be in groups minimum of 4 for the larger types and minimum of 6 for the smaller types.
Your tank is not cycled and it's needs to be with that stocking, with that amount of ammonia being produced, ideally you should be doing daily water changes to keep it minimal. Cycling is a process in which you grow nitrifying bacteria that breaks down harmful substances such as ammonia, and converts them into less harmful ones. Ammonia gets converted into nitrites by nitrosifyers, such as Nitrosomonas, and nitrosococcus. Nitrites are just as Lethal as ammonia, so ideally both levels of these should be 0 ppm. Nitrites thn get converted into nitrates by nitrobacter. Nitrates are as lethal as ammonia and nitrites so this level doesn't have to be around 0 ppm as ammonia would be, a good level of nitrates would be around 10-15 ppm. Indicating that you have a cycled tank.
If he looks bloated then it could be a result of over feeding, swim bladder disease or constipation, usually over feeding and constipation doesn't cause eye fish to lay on its side at the surface and cause eradicated swimming, I would diagnose this as Swim bladder Disease
This could possibly be a symptom of Swim Bladder Infection which is usually caused by over feeding. My recommendation towards treating this would be along the lins of introducing a dose Of Epsom salt at around 1 teaspoon per gallon
- eradicated swimming
- floating towards the surface of the water
- trouble swimming up
- seems to be doing "rolls"
Treating this usually non fatal illness will be very easy to do. Usually a dose of aquarium salt, something along the lines of 1 teaspoon per 3 gallons for tetras, and 1 teaspoon per gallon for more salt tolerant fish such as Bettas. Typically after 1-2 Epsom salt treatments it will go away. Ideally during this treatment time you would also want to offer peas, and daphnia to the fish as this will also help
This illness is usually caused by incorrect feeding with diets that have little or no nutritional value, or over feeding. Or incorrectly feeding more than the needed periods in the day to feed, like feeding very often
This illness is easily prevented by going along the lines of not over feeding, providing a varied diet with many good foods, and avoiding feeding lots of low nutritional value foods