Gas Exchange in Marine Aquariums: The Importance of Proper Degassing

Introduction

In marine aquarium keeping, there are many opinions about the necessity of aeration and degassing. Some believe that a protein skimmer alone is sufficient. Indeed, the skimmer was introduced into marine systems as a kind of replacement for degassing systems, and to some extent this is correct, but there are important nuances that need to be understood.

A skimmer can never be a complete degasser, and this has long been understood by all experienced aquarists. The reason is simple.

What is Degassing and Why a Skimmer Isn't Enough

Degassing is the process of removing certain gases from marine water.

Air is a mixture of different gases: oxygen, carbon dioxide, and others. We need to understand what we want to introduce into the water, and what needs to be removed.

Important point: oxygen is lighter than carbon dioxide. Carbon dioxide tends to sink down. Most aquarists have their skimmer installed in the sump, at the bottom. If the intake tube isn't extended higher than one meter, the skimmer will draw in air saturated with carbon dioxide and return it back into the system, oxidizing the water and lowering the pH.

The normal pH level in a marine aquarium is about 8.2-8.4. At night, a drop to 8.0 is acceptable, but if the value falls below that (to 7.8 or less), these are serious fluctuations that negatively affect the inhabitants.

Why the Sump is Often Better than the Display

An interesting feature of many aquarium systems is that conditions in the sump are often better than in the main aquarium (display). I call such systems "flipped" — everything is bad in the display, but good in the sump.

This happens because:

• The sump itself has its own "degasser" (skimmer)
• With less volume, water flow in the sump is greater than in the display
• Powerful current creates water movement, preventing stagnant zones

This can manifest in a paradoxical situation: fish sick with Cryptocaryon and Oodinium, moved from the display to the sump, recover, but when returned to the display get sick again.

Practical advice: place control sensors (pH, ORP) in both the display and sump, monitor the difference in readings throughout the day. If there's even the slightest difference, it means the flow through the display isn't strong enough. It's even better to use an oxygen sensor — you'll immediately see that there's more oxygen in the sump than in the display.

Principles of Proper Gas Exchange Organization

1. Proper Water Exchange Between Display and Sump

The first and most important thing is to organize powerful water exchange:

• Return pump selection: For a 500-liter aquarium, you need a pump of at least 50,000 l/h (flow rate = aquarium volume × 10). The problem is that the stated power often doesn't correspond to the actual performance. A 5,000 l/h pump often delivers no more than 3,000 l/h due to insufficient pressure and impeller design.
• Solution: It's better to choose a pump with a reserve, for example, 10,000 l/h for a 500-liter aquarium. The power of such a pump can be slightly reduced, but it will be able to provide the necessary pressure.
• Correct pipe diameter: The return pipe (from pump to display) should match the diameter of this pump's output opening. Any narrowing or additional bends will result in power loss.
• Skimmer placement: Place the skimmer before the return pump compartment. The sequence should be: Skimmer → return pump compartment → display. This way, water enriched with air immediately enters the display.

2. Proper Current Organization in the Aquarium

For effective degassing, mixing of all water layers is necessary:

• Current direction: Water should mix not only horizontally but also vertically. Corals live in variable water movement — with fluctuations and different flow strengths.
• Current diversity: Monotonous current allows dirt to settle in certain places. If you set up waves differently (using wave generator pumps), dirt won't settle but will gradually be removed from the display into the sump and filtered out.
• Wave strength: For effective washing of coral colonies, the wave height should be at least 1 inch (2.5 cm). Imagine an acropora colony 10 cm wide — for water to pass through it, the difference must be sufficient. A wave fluctuation of 1.5 cm is too small and won't provide effective coral washing.

3. Elimination of Stagnant Zones

Stagnant zones are places where harmful gases and detritus accumulate:

• Dense coral colonies: Inside an Acropora colony with numerous branches, water stagnation often forms and detritus accumulates. Water should pass through the entire colony first in one direction, then in another.
• Under substrate: Large coral rubble with detritus underneath creates a stagnant zone. The more densely dirt settles, the less ventilation there is, which worsens dirt accumulation.
• Grottos and caves: Fish look for places with minimal current for sleeping. Detritus and harmful gases, including ammonia, accumulate there. In fact, fish sleep in "ammonia pits," which explains why they have cloudy eyes or white rash on their skin in the morning.

4. Degassing Chamber and Its Adjustment

The chamber into which water flows can perform the function of degassing if properly configured:

• Operating principle: When water falls and bubbles, gas evaporation occurs. But this isn't just aeration — it's important that large volumes of water mix.
• Intensity regulation: The strength of water bubbling can be regulated using valves. In emergency situations (e.g., low pH or ORP), open the valve for enhanced degassing. In normal mode, you can partially close the air inlet to reduce noise.
• Mistake: Many believe that light water bubbling is degassing. However, if all the water in the degassing compartment doesn't mix, the effect will be minimal.

Air Source for the Skimmer

A critically important point: the air that the skimmer draws in must be clean and free of CO₂!

• Room air problem: If there's a lot of carbon dioxide in the apartment (from people breathing, cooking on gas), the skimmer will drive it into the water.
• Solution: Extend the skimmer's intake tube outside or into a well-ventilated area.
• Paradox: Some notice that without enhanced "degassing," pH is higher than with it. This happens when the skimmer draws in air with high CO₂ content and drives it into the water.

Signs of Ineffective Degassing

• Fish with cloudy eyes in the morning: Which clear up by evening — a sign of gas poisoning in stagnant zones at night.
• Fish staying near the surface: Their breathing is slightly accelerated, although there's no obvious suffocation — they're trying to move higher away from harmful gases.
• Uneven coral growth: In certain areas of the aquarium, corals don't color up and grow poorly, although there's sufficient light throughout the aquarium. These are "dead zones" with poor gas exchange.
• Fish dying in the morning: Often for unclear reasons — this could be gas poisoning in a stagnant zone where the fish hid for the night.

Aquarium Surface and Gas Exchange

Never cover the aquarium surface: it should be open for natural degassing. The degassing process is similar to the cooling process — it occurs through the surface layer during evaporation.

Comparison with Nature

To understand the importance of gas exchange, a comparison with natural conditions is useful:

• Depth-to-area ratio: In the ocean, depth is incomparable with surface area. Even the Mariana Trench is only 12 km deep.
• Natural degassing: Despite enormous volumes, the sea also has stagnant zones with detritus accumulations and hydrogen sulfide, but the surface layer is always clean, especially in coral reefs that are well-lit and washed by waves.
• Coral habitat depth: Corals live at depths up to 20 meters, where light is still intense enough and harmful gases don't yet accumulate.

Cleaning Corals with Air Bubbles

A separate topic: this is a useful process of cleaning corals with air bubbles. It's recommended to conduct it several times a week or even daily (read the following articles).

Proper Reef Organization

For effective degassing, it's important to properly organize the reef itself:

• "Floating" reef: The reef should be installed on stands so that water can wash it from below. This will prevent gas stagnation in the reef rock grottos.
• Washing all layers: Water should be pulled through the reef in different directions, mixed across all layers (bottom, middle, surface), lifting detritus and gases from all grottos.

Conclusion

Degassing is not just a component of the marine ecosystem, but a complex process:

• Without degassing, long-term maintenance of a marine aquarium is impossible
• It's as important as the presence of water — without water, marine inhabitants will die quickly, and without degassing slowly but inevitably
• Degassing is not just bubbling, but comprehensive mixing of the entire water column across all layers and directions
• The quality of air being driven into the water is no less important than the degassing process itself

Remember: if someone claims they "don't have a degasser," they don't understand that degassing in one form or another is present in any system. The question is only in its effectiveness and proper organization.