Filamentous Algae in Marine Aquariums: Causes and Control Methods

Introduction

Filamentous algae are thin, hair-like algae that spoil the appearance of aquariums and cause many headaches for aquarists. To effectively combat this problem, it's necessary to understand the causes of their appearance and create conditions in which they cannot actively develop.

Main Causes of Filamentous Algae Growth

Low pH

The main cause of filamentous algae development is low pH. When pH drops below 8.1, filamentous algae receive favorable conditions for growth. However, it's important to understand that low pH is a consequence of deeper problems in the system.

Low pH can be caused by:

• Weak biological filtration
• Large amounts of organic matter and overdosing of certain microelements
• Low carbonate hardness (KH)
• Excessive CO2 in the water

Excess CO2

All algae, including filamentous types, significantly accelerate their growth with carbon dioxide in the water. In freshwater aquariums, CO2 systems are often used to stimulate plant growth — under such conditions, algae growth can accelerate up to 10 times.

Excess CO2 in a marine aquarium can appear due to:

• Poor room ventilation
• Inefficient skimming that "hammers" CO2 from the air into the water
• Stagnant zones where decay processes release CO2
• Insufficient water circulation between the display and sump

Disruption of Biological Balance

Filamentous algae appear when the biological balance is disrupted. A normally functioning aquarium biology is a direct competitor to all lower algae, including filamentous types.

Algae gain an advantage when:

• Biofiltration is weakened
• Nutrient consumption by beneficial microflora is disrupted
• There are stagnant zones with decaying organic matter

Nutrients and Filamentous Algae

Connection with Nitrates

Filamentous algae actively consume nitrates. Often, you can observe a decrease in nitrate levels during active growth of hair algae. Under certain conditions, filamentous algae can thrive even with relatively low nitrate levels.

Dependence on Phosphates

Unlike some other algae (such as cyanobacteria or dinoflagellates, which often appear when phosphates are zeroed out), filamentous algae can grow with minimal phosphates, primarily using nitrates as a nutrient source.

Methods to Combat Filamentous Algae

Raising pH Naturally

Raising pH to 8.3 significantly slows the growth of filamentous algae. It's important to achieve pH increases naturally, not artificially:

• Improving biofiltration
• Strengthening biological filtration
• Dosing carbon (if natural nutrition is insufficient)
• Developing beneficial bacterial colonies

Maintaining Optimal Carbonate Hardness

• If pH is low due to low carbonate hardness, it's necessary to increase KH
• The optimal KH level in a marine aquarium is 7-9 dKH

Improving Water Circulation

• Eliminating stagnant zones using flow pumps
• Increasing return pump power
• Increasing pipe diameter for better water flow
• Improving circulation between sump and display

Effective Protein Skimming

• A quality skimmer removes organics and saturates water with oxygen
• Higher oxygen content means fewer conditions for acidification

Biological Control Methods

There are several effective organisms that naturally control filamentous algae growth:

Sea Hare (Dolabella)

• The most effective "mower" of hair algae
• A brown-colored mollusk that quickly eats filamentous algae
• Requirements when choosing: select active specimens that move and feed
• If natural food is scarce, can be fed moistened nori sheets (3 times a week)
• Leaves large feces, which may require additional filtration

Snails

• Cypraea annularis (Gold Ring or Money Cowrie) — specializes in filamentous algae
• Turbo and Trochus — also good at eating hair algae
• Limitation: snails are only effective against young filamentous algae (up to 2 mm long)

Hermit Crabs

• Small hermit crabs effectively consume filamentous algae
• Important: don't give them large shells, so they don't grow too big
• Small-sized hermits can create an effective "army" for fighting hair algae

Sea Urchins

• Diadema — good at consuming filamentous algae
• Astropiga radiata (red urchin with blue fluorescent dots) — excellent option
• Some urchin species may occasionally show aggression toward their own kind
• The "watermelon" sea urchin is also effective, but tends to collect small stones, debris, and coral frags on itself

Interesting Observations About Filamentous Algae

Age Characteristics of Aquarium Systems

Filamentous algae almost never appear in new systems. For their development, an aquarium must be in a "stagnant" state for a long time (usually about a year, although sometimes six months is enough). This is related to the accumulation of certain metabolites and the creation of suitable conditions for hair algae development.

Comparison with Natural Environment

In natural marine ecosystems, filamentous algae are rare. They can be found in some shallow areas with stagnant water (for example, in the Caribbean Sea), but they are not characteristic of most coral reefs with active water exchange.

Conclusion

Filamentous algae are an indicator of certain problems in the aquarium, related to disruption of biological balance, low pH, and often stagnant zones. The key to fighting them is not so much mechanical removal, but creating conditions in which they cannot develop.

Proper biofiltration, good water circulation, an effective skimmer, and maintaining pH above 8.3 by natural methods are the main steps to getting rid of filamentous algae. Combined with biological "helpers," these measures will not only eliminate hair algae but also prevent its appearance in the future.