For a FW
or BW tank a wet/dry system using bioballs is great. Most of the bioballs are above water (dry) and those on the bottom are always in water (wet). Above the bioballs is usually a diffusion plate ( a plate or box with lots of holes drilled in it). On top of the diffusion plate is a filter pad.
Water from the tank flows over the filter pad to catch the larger particles. The diffusion plate gets the water to trickle over the bioballs. The filter pad needs to be cleaned or replaced often.
The water flowing over the bioballs allows lots of air to come in contact with the bioballs. That's great for the bacteria that turn ammonia to nitrite and nitrite to nitrate. Nitrate is then removed by partial water changes (PWCs).