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To start, a good staple diet for Corydoras are shrimp pellets. They are healthy, and sink right away. Look for the HBH brand, they make good quality ones (stay away from Wardley shrimp pellets, they have been known to cloud water and they get very messy).
A good ratio is two shrimp pellets a day per Cory, but if you have other fish, like cichlids, that may eat them before the Corydoras can get them, give your Cory at least four.
Then, start adding algae/veggie wafers every second day. Try to get the Hikari Brand, they make excellent Cory food. It gives the Cories some vegetable matter, which is essential in their diet.
Hikari Sinking Wafers (a Cory favorite) is even better for your Corydoras then shrimp pellets or algae wafers. So if you can pay a little more, I would highly recommend getting these too.
Hikari Bio-Pure Freeze-Dried Tubifex Worms fit the bill perfectly. They are cheaper then live or frozen foods, but in my experience Corydoras like them more. They come in small cubes, and the problem is they float so Corydoras can't get them easily. But, there is away to get them to sink. Wrap some aquarium metal weigths (plant anchors work well) gently around the cube, then throw it in the tank and watch a feeding frenzy begin at the bottom of your aquarium.
Check his gills inside, are they swollen? If you dont want to do it as I described as it is a bit nerve wrecking and dangerous if done wrong give him a large piece of food in onc and look closely inside his gills whilst he is eating, if they seem swollen u have SFH