I have a 2.5 and 3 gallon NPT. I have never used filters on them and they are doing well. There isn't any kind of water movement except what the fish or shrimp create. In my 12 gallon I use a filter without any media (except a little polly-fill, but that's mostly to keep the water from splashing around), but it is mostly for my fish as they acted better with it running. When it wasn't running the corydoras weren't eating or being as active.
From your other post, I believe you are working with a 2 gallon tank. I would say you are fine without a filter. The first few weeks to months you may need to do frequent water changes, but once it is settled and balanced, these are great little tanks. I love them.
I posted this on your other thread, but here is an article by Diana Walstad about her 1 and 2.5 gallon tanks