I boiled mine for 20 minutes and let them dry for a day or two and they were fine. I don't plug the holes. My bettas have no trouble with the holes. If they have short fins, they go through. Long fins, then they don't. Here's two pics of my 20gL. This tank is a paradise for my fish, especially my kuhli loaches! lol I have 4 female bettas in there, but I have terra cotta pots in all of my 4 tanks.
[Apologies for the sorry state of my tank. lol I need to do a water change and I NEED my plants to start growing back. It used to be plant haven in here!]