cory help

Hi, I'm new to fishkeeping and have been loving it, however I am worried about one of my Corys. I have 3 in a 57 litre (12.5g) tank along with 6 neon tetras, 4 danios and a dwarf gouramis. A few weeks ago the smallest Cory disappeared and I couldn't find him. A few days later I found him stuck...