Problem in community tank?

Your tank volume isn't even near 10 gallons and you already have about 20 inches if fish in it. The consequence is the death of your fish. I calculated the volume and it's only 7.06 US gallons. This is very small for a community tank. The best thing for you to do right now is keep only up to 7 of the fishes that you like the most and take the others back to the pet store to avoid any more deaths. You should skimp on the corydoras because they can grow up to 3 inches(depending on the species). Feed lightly and provide excellent filtration and you'll find your fish living up to 3 or more years.