Fully submersible (internal) filters are perfect if you have a 'dead' spot in your aquarium where water flow needs to be increased, or if your tank carries a high bioload and is in need of supplemental filtering. They are also well suited for low water habitats, such as paludariums.
1. Duetto
What separates this internal from others is an adjustable aeration feature that mixes air with the water for improved gas exchange, as well as two output adapters which allows for horizontal or vertical placement.
2. Fluval
This small but powerful filter allows for a choice of filter media, rarely clogs, and is very affordable. Like all internals, the primary downside is that cleaning is cumbersome due to the need to remove it from the aquarium.3. Proquatics
The Proquatics offers a wide range of flow rates, and a sponge filter that promotes biological filtration.