I don't mean to make you feel bad :( I feel like I failed too.. I could have caught this earlier if I asked a few more questions. :( Everyone makes mistakes when they first start fish keeping, and most people will kill at least one fish just because they don't know any better. IMHO, keeping fish is harder than keeping any other house pet because they are just so sensitive to their environments, and there's just a lot to learn. But you have a good heart and I don't mean to discourage you. I'm sorry.
It sounds like your guy is really bad right now.. Worse than I had thought.. The best thing you can probably do at this point is get him out of the 20 gallon tank water into something smaller, so he can more easily reach the surface. I'd do a 100% water change into thie container with identical temperature water and still acclimate very slowly. Also you will be able to make very frequent water changes in it, and it will aleviate any possibility that something in the 20g is off and making your fish sick. It will have to be something you can keep warm though.. Either if you have a smaller tank you can put a heater in or an acrylic bowl of around 2 gallons that can be filled half full and floated in the larger tank... something around 1-2 gallons or so (wider is better than taller) would be good. In the bowl you can also choose to put a plant that are tall enough to reach the top with large flat leaves he can lay on, making it even easier for him to get to the surface. Something that looks like the leaves on this: http://www.amazon.com/Hagen-Marina-N.../dp/B005JW5VHQ
. To relieve the stress of the fluid build up on his organs use epsom salts - you know the drill - pure and predissolved. I would go ahead and go on up to 3 tsp this time. Add very slowly. I would do 1 tsp over and hour then give him a few hours break and add another slowly, etc. This will not cause him distress at all. It will help relieve some of the pressure and while it probably won't cure him it might make him feel better.
In this bowl you can make 100% water changes every other day, readding the epsoms - once he's used to the full dose you don.t have to keep adding it back in slowly.
This is how to do a large change without shocking:
To do a water change, use a little cup like a plastic solo cup - this cup must be only for him and have never been used with soap or other chems. Scoop him up in this cup (keep him in the cup about 1/4 full of water - it doesn't need to be much because he won't be in it for long) and leave him in the cup while you change his water. To do the 50% use a turkey baster - dedicated only to him that has never seen soap or chems - and drag it through the gravel and try to suck as much of the poop out as possible, in addition to 50% of the water. Use a thermometer under the running tap to get it to be the same temp as the water that is normally in his tank. When the thermometer says the flowing tap is the right temp, fill back up his tank. At this point, add the conditioner (dose for how much water you change - if you change half the water you add half gallon worth of conditioner, If you do a 100% water change dose for the full gallon change). Float his plastic cup with him in it in the new water. Slowly add a couple tablespoons of the new water into his cup every 10 minutes for at least an hour. Finally, dump him in gently but try to get as little of the old cup water back into the tank as possible. When you do the weekly 100% you will do mostly the same thing except empty his tank fully and rinse everything in it very well under warm water but never use soaps or chemicals. Once it's fully cleaned/rinsed you can refill it and repeat the cup/acclimate phase.
You may already know this but the key to the water change is quickness in the actual change, slowness in acclimation, and the keeping of the water as similar as possible.
In addition to this.. if you want to try to save him he'd need an internal antibiotic.. how long was he on the maracyn plus and how long ago? treatment with this should have been every other day for 2 weeks. Along with redosing should have been 50% water changes and then 100% water change at week's end. I'm not sure what else you have around but few else stands a chance. Kanaplex is one. Furan 2 stands a long distant third. I wouldn't bother with anything else.
Just for future note, flakes tend to cause constipation issues in fish. Some are more prone to constipation than others - some can spend their whole lives on flakes and never have a problem and some can't even have a pinch or it's all downhill. This is why I suggest feeding a high quality pellet like New Life Spectrum Betta (feed 5-8 split into 2 or 3 smaller feedings) or Omega One Betta Buffet 3-4 split into a couple feedings. They tend to cause the least amount of constipation issues and they seem to have the best results as far as overall fish health. Also anything freeze dried must be thoroughly soaked for at least 10minutes or so in a cup with your liquid of choice - garlic guard, vitachem or even just a bit of tank water. They are very dry and tend to cause constipation if you feed dry. No matter what you choose to feed make sure they get one fast day a week off all food.