It isn't a good idea the use no filter in a tank. Unless maybe it is a plant-only tank. ![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I'd love to see a picture of this tank.
You see, it doesn't quite work like that. In nature the water is indeed filtered and goes through the same cycle that a filter replicates in your tank. I think an air driven sponge filter would work great in your situation. Although I'm still a little confused as to exactly what you have going on here.
If you have no equipment, you will have nasty stagnant water and your fish will die. It's impossible to exactly replicate a pond, lake, river, or stream that fish naturally live in with a 6 gallon tank..... or any tank for that matter.
Truth is, we use equipment like filters and heaters to do exactly as you intend and that is to mimic how they live naturally by creating proper temperature,water quality, and so on. Without this equipment, we cannot provide our fish with the environment that they live in naturally. Without any equipment it's equivalent to a puddle, not a small pond.
If you can't use a filter, then have lots of plants in the tank, especially floating plants. They will use a lot of nutrients and encourage small insects to land on the water. If the tank is outdoors, you might get dragonfly larvae in the tank and they will eat fish so monitor it and remove any insects that pop up in the tank.
It isn't a good idea the use no filter in a tank. Unless maybe it is a plant-only tank.![]()