Reef Aquarium Without Fish: Maintaining Biological Balance

Maintaining a marine reef aquarium without fish might seem like an attractive idea for those looking to avoid the hassle of feeding fish, or those who simply prefer the aesthetics of a clean reef with corals. However, this approach involves many nuances and potential problems that are important to understand in advance.

What is a "fishless" aquarium?

A "fishless" aquarium is defined not so much by the absence of fish as such, but by the absence of regular feeding. Even if you have several small inhabitants, such as goby shrimps or small blennies that feed mainly on algae or detritus, and you don't add food daily – such a system can be considered "fishless."

It's important to understand that even a small number of fish in a large volume (for example, three fish per thousand liters) or a complete absence of fish creates the same problem – a low biological load, which disrupts natural processes in the aquarium.

Biological cycles and their importance

Two fundamental biological processes underlie any healthy marine system:

1. Nitrogen cycle (nitrification) – the conversion of ammonia to nitrite, and then to nitrate with the help of bacteria.
2. Denitrification cycle — a biological process in which bacteria convert nitrates (NO₃⁻) into nitrogen (N₂) or nitrogen oxides (N₂O), removing them from the water.

In an aquarium with fish, ammonia constantly enters the system through fish gills, their excretions, and the decomposition of food residues. In an aquarium without fish, this source of ammonia is absent, which creates a problem – the natural nitrogen cycle is interrupted.

When the nitrogen cycle is interrupted, the system becomes vulnerable to colonization by dinoflagellates (harmful unicellular organisms), which can take on the role of primary filtrators. This leads to a complete disruption of the biological balance, the appearance of unpleasant odors, and coral death.

Solution: artificial maintenance of cycles

To maintain a healthy environment in a fishless aquarium, it's necessary to artificially provide the substances that fish and their food would normally supply. There are several approaches:

1. Creating a "leaching" structure

It's important to create a zone in the aquarium where natural ammonia release will occur. For this, you can use:

• A layer of sand with organic filling (detritus), where slight decay and ammonia release will occur
• Live rocks with rich bacterial flora
• Bioceramic material from an old, established aquarium

Such a structure will constantly "leach," meaning it will release small amounts of ammonia, supporting the nitrogen cycle. If detritus stops falling out of live rocks, it's a sign that the bacterial flora is degrading – bacteria have stopped reproducing.

2. Regular coral feeding

Even if you don't have fish, corals need nutrition. Regular feeding of corals (once a day) not only nourishes the corals themselves but also maintains bacterial flora. It's important not to skip feedings – even one missed day can disrupt the fragile balance of the system.

Coral feeding provides:

• Input of organic matter, which is partially converted to ammonia
• Nutrients for bacteria
• Maintenance of nitrogen cycle stability

3. Dosing nutrients

In extreme cases, you can use dosers for direct introduction of ammonia, nitrates, and phosphates. However, this approach has significant drawbacks:

• It's difficult to determine exact dosages
• You lose the ability to monitor natural processes (if you dose nitrates, you cannot use their level to judge the operation of the nitrogen cycle)
• There's an increased risk of errors that can lead to coral death

4. Monitoring biological processes

It's important to control that the nitrogen cycle is working properly. If the nitrate level is stable and doesn't drop to zero, it means there's enough ammonia to maintain the cycle. If nitrates begin to decrease, it's a sign that ammonia is becoming insufficient and the cycle may be interrupted.

You should also control the phosphate level, which is necessary for the denitrification process – the conversion of nitrates by bacteria.

Live rocks – the foundation of a fishless system

For a fishless aquarium, it's critically important to use live rocks – rocks with already developed bacterial flora. Unlike an aquarium with fish, where you can start with "dry" rocks and gradually develop bacterial flora on them, a fishless aquarium on dry rocks is practically impossible to successfully launch.

Live rocks provide:

• Ready bacterial flora
• The possibility of quickly starting biological cycles
• More stable biological balance

When using live rocks, it's important to maintain their biological activity by adding nutrients and monitoring signs of bacterial activity (for example, by observing detritus falling from Live Rocks).

Problems and risks of a fishless aquarium

Maintaining an aquarium without fish is associated with a number of significant risks:

1. Instability of biological cycles. Without a constant source of ammonia, the system remains extremely vulnerable to disruptions.
2. Difficulty in diagnosing problems. In an aquarium with fish, fish often act as indicators of problems, reacting to changes in water. In a fishless aquarium, you may notice a problem only when corals have already begun to suffer.
3. Rapid system degradation. When the balance is disrupted, a fishless system can die very quickly, turning into a "swamp" with dinoflagellates.
4. Need for constant monitoring and dosing. More careful monitoring of water parameters and regular nutrient introduction is required.

Practical recommendations

If you still decide to maintain an aquarium without fish (or with a minimal number of them), follow these recommendations:

1. Start with live rocks. This is critically important for the successful launch of the system.
2. Don't skip coral feedings. Even if it seems that corals don't need food, regular feeding is necessary to maintain bacterial flora.
3. Create a sand substrate. Sand with organic filling will be a source of detritus and help maintain the nitrogen cycle.
4. Control nitrate and phosphate levels. These parameters should be stable and not drop to zero.
5. Make changes gradually. Any drastic changes can disrupt the fragile balance of the system.
6. Consider alternatives. Perhaps the best solution would be to add a few small fish that will help maintain natural biological cycles without creating excessive load.

Conclusion

Maintaining a reef aquarium without fish is possible, but requires a deep understanding of biological processes and constant attention to the system. This is, in essence, an experiment for experienced aquarists, not a simple solution for beginners.

Before deciding on a fishless aquarium, weigh all the pros and cons. Perhaps a more practical solution would be to keep a minimal number of fish that will help maintain natural biological cycles but won't create a significant load on the system and won't require frequent feeding.

In any case, the key to success will be creating a stable biological system with active bacterial cultures and maintaining a constant, albeit minimal, supply of nutrients to support the nitrogen cycle.