CO2 Distribution and Water Flow in a Nano Tank - Preventing Dead Spots

When discussing planted aquariums, the focus is often placed on stable CO2 injection, proper lighting and fertilization. However, water circulation is just as important. Without adequate flow, so-called "dead spots" can develop, areas where plants receive less CO2 and nutrients, often leading to algae outbreaks.

In this case, the aquarium is an ADA Mini M with a volume of 20 liters. The filter outlet, inlet, and CO2 diffuser are all positioned on the front right side of the tank. The diffuser is placed directly beneath the filter intake, allowing the CO2 bubbles to be drawn into the filter and completely dissolved inside an OASE FiltoSmart Thermo 100 canister filter, rated at 600 L/h.

Given the small size of the aquarium, a conventional lily pipe would have produced excessive flow. To create a gentler and more dispersed current, a spin lily pipe was chosen instead. This design softens the outflow and provides a more subtle water movement throughout the aquarium.

After some time, however, a clear difference became noticeable between the two sides of the tank. Plants on the right side, closer to the filter outlet, showed stronger growth and remained free of algae.

In contrast, the left side of the aquarium developed fuzz algae, indicating insufficient circulation and poor CO2 distribution in that area.

To improve water movement throughout the tank, the spin lily pipe was modified. A small hole was carefully drilled into the glass ring opposite the outlet tube that feeds water into the spinning chamber. This simple modification redirected part of the flow toward the area that previously received little circulation.

The result was significantly improved water movement across the entire aquarium, more uniform CO2 and nutrient distribution, and the elimination of dead spots. Following the modification, growing conditions became much more consistent throughout the tank, leading to healthier plant growth and a reduction in algae-related issues.

This example demonstrates that in small planted aquariums, overall filter flow is not the only factor that matters. Equally important is how that flow is distributed within the aquarium. Sometimes, a simple modification can dramatically improve circulation, CO2 distribution, and the overall health of the aquascape.