Will your fish store take them back? Or can you keep the betta in the 10, and get a bigger community tank. If you get a 20 long you can have a lot more corydoras and a few more tetras.
Think that's hole in the head disease. Not sure how to treat, but pretty common if you have poor water quality, do you know your parameters?
@Metalhead88 @Wills
Thanks Charlie. I looked at my filter thing to try and figure out how to take the carbon off. It's actually attached to the sponge, I'll attach a pic later. Should I cut it off?
Lime buildup means you have VERY hard water. Which is good, because that's what fish like mollies and platies need. You should look into getting a bigger tank because mollies need a lot of space, as do platies. You can get a old credit card and scratch the lime off.