Poor guy, he looks very ill. Do you have a water test kit and do you know if your tank has undergone the nitrogen cycle?
I googled QuickFix, but I couldn't find the product or a list of ingredients. Can you tell me what ingredients are in the medication? If I had to guess, it's probably a mixture of malachite green, formalin or acriflavine, and maybe some random antibiotic.
My initial impression of this fish is that if he was fine yesterday, and is clamped, lethargic, and barely moving right now, it's likely a systemic bacterial infection--systemic infections hit hard and they hit very fast. If the fish had something else, he'd probably look a lot better right now, and he doesn't look bloated, which would be the only other explanation for his sinking.. Unfortunately systemic infections don't have the best prognosis--but if the fish is still eating, you might be able to turn him around with the right medication.
Is the heater adjustable? If it is, I would slowly turn the heater down a half a degree per hour until the temperature is at 76 degrees. Bacteria thrive in warm temperatures, so this should slow it down a bit. You should try to lower the water level so that he doesn't have to work hard to get to the surface to breathe.
I would look for a more serious broad spectrum antibiotic, such as a combination of both Maracyn I and Maracyn II at the same time, API's Triple Sulfa, or a Kanamycin medication such as Seachem's Kanaplex. I always recommend that anyone who uses an antibiotic read this article: http://edis.ifas.ufl.edu/fa084
so that you understand how these medications work and the risks involved with working with them. If you treat him with an antibiotic, you should use a different container--I use sterilite/rubbermaid plastic storage bins, since they're available at walmart or target in 4 gallon sizes for only about $3 each. They're safe to heat and house your fish in and you can't beat the price. If you have a large tupperware container that will work too. Try to pick a container that is wide and shallow so the fish won't have to strain to reach the surface, and make sure it has an even number of gallons so that the medication will be easy to divide and dose correctly. If you don't have one, it would be a good idea to get a small air pump and air stone because antibiotics will remove oxygen from the water. Tie knots in the tubing or buy a valve to restrict the bubble flow so that the current doesn't disturb your betta.