Please post a picture. Kind of curious to know what filter you are using. Your tank is way overstocked. Regardless of what anyone says, or what you observe, your fish are NOT happy. They are probably surviving depending on if you are doing very frequent water changes. Cories need groups, not 1, not 2, or even three in my opinion. They need at least 5. Ottos usually prefer to have a small group of themselves too, and if you do have more they will be a bit more outgoing and not so skittish. Think of the dimensions of your tank...12″ x 6″ x 8″it is the size of a ruler, man. Bettas are also quite aggressive. Many people keep them in community tanks, including myself at times...BUT you need to make sure there is room for the other fish to hide or room for the Betta to get some alone time. What kind of plants do you have? Some plants can really help your nitrate levels, but if you get an ammonia build up it will kill everything in there.
That being said, this is not meant to be a mean or rude post. This is meant to be helpful to you. You'll find that the people on this forum care about their fish. They also want to help fellow hobbyist. It is exciting to have a lot of fish. Different types that do different things, colors, sizes and shapes. But at the end of the day you need to consider that what you have is a living creature. It can be a fun hobby, and very rewarding too, but please consider your pets. If you don't care about them that might be an indicator that this is not the hobby for you. I know that overstocking small tanks is inevitable...but this is just too too too much. Honestly just keep the Betta and get a snail OR some ghost shrimp OR buy a new tank. Rehome everyone else. Every petstore I have been to in my area is having $1 per gallon. A ten gallon tank is only $10, better yet a 20g long is great for planting. Only $20. If you have a decent filter there you go.
We are trying to help you here. No one on this site will encourage something that would harm the animals or plants we love.