Substrate and Water Flow in Marine Aquariums

Introduction

This guide covers the interrelated topics of choosing and maintaining substrate in a marine aquarium, as well as organizing proper water flow. These issues are closely connected, just like salinity and temperature discussed in previous materials.

Sometimes it may seem that certain topics are repeated in different guides, but here they are examined from a different angle and with new nuances. This is necessary for complete understanding, as many aspects of marine aquarium keeping are interconnected.

Is Substrate Necessary in the System?

Among aquarists, there are different opinions: some successfully maintain aquariums with substrate, while others prefer systems without substrate. You often hear that "each system is individual," but the problem is that without understanding all the processes, people simply can't see that everything always develops similarly and ultimately leads to common denominators and results.

If systems were truly individual, their basic parameters could differ significantly: salinity could be 25 ppt instead of 35 ppt, temperature 65° instead of 75°F, pH 9 instead of 8.3. But such deviations are impossible for marine coral life. You can certainly create a coldwater aquarium, but it would house completely different animals.

Someone who claims their system is "individual" would need to move parameters outside the "green zone" while still successfully growing corals. Only then could one say the system truly differs from others. In practice, we see that successful systems always remain within an acceptable range of key parameters.

A system's individuality can only manifest in nuances, where some parameters compensate for others while all remain in the acceptable zone. For example, increased salinity with simultaneously increased temperature - one parameter compensates for the other. The same happens with decreased salinity and decreased temperature. But if you increase salinity and decrease temperature, a crisis may occur, even if each parameter individually was in the acceptable range, you'll see a clear deviation from the norm.

This is why many aquarists, not understanding the processes, encounter problems. A seemingly simple question: "add substrate or not," but out of dozens of beginners, only a few succeed normally, the rest face problems, and 10% lose everything in the first year. They invest money and then abandon the hobby, deciding "it's not for them," when they simply didn't follow some simple rule they underestimated.

Substrate as a Biofilter

When deciding to use substrate, you need to understand that substrate is not just a decorative element to improve aesthetics. It's a full-fledged biological filter that requires constant care. It's a "living entity," a colony of microorganisms requiring attention even more than other aquarium inhabitants.

The choice to use substrate or not should be based not on aesthetic preferences, but on your readiness to provide proper bioload and care. Substrate is an additional biofilter, a kind of "global entity," a colony of microorganisms requiring constant attention.

Advantages of substrate:

• Stabilizes water parameters

• Provides additional biofiltration

• Many useful elements are produced in the substrate which, when released into the water, stabilize parameters and provide nutrition for corals

• Parameter stability is critically important for marine aquarium health

Types of Substrate and Their Applications

The first criterion to consider when choosing substrate: will it be a biofilter or just a decorative element?

1. Decorative Sand (not a biofilter)

• Fine, powder-like sand (fractions of a millimeter)

• Thin layer (no more than 1.5 cm)

• Needs constant stirring to prevent compaction

• Should not accumulate in thick layers under the reef base

The problem for many aquarists: sand accumulates under the reef (up to 2 inches), while at the edges it's already washed away to the glass. At first, a person levels it once a week, then abandons this task, and uncontrolled processes begin there. Decorative sand, which in principle cannot be a biofilter, is forced to perform these functions. This can be compared to someone who comes to the gym for the first time and tries to lift a 200 kg barbell - at best they'll injure themselves and end up in the hospital. Similarly with sand - it can't handle the load, "goes bad," and poisons the system.

Important: If you decide to use decorative sand, the layer should not exceed 1.5 cm. If you end up with 5 cm in one corner and 0 in another, you need to level it. If you cannot or are too lazy to do this regularly - it's better to go without sand altogether. If the current blows sand into piles, and you can't provide proper flow or take responsibility for weekly leveling - give up on sand. Hoping that "it'll work itself out" somehow is pointless - the sand will simply go bad and destroy your aquarium's ecosystem.

2. Biofiltering Substrate

• Fine coral rubble (1-5 mm)

• Layer no more than 1 inch

• Water should pass through it, detritus should fall between particles

• Better to use 3-5 mm rubble so it's "ventilated"

• Doesn't require such frequent overturning as decorative sand, but once a month check for hydrogen sulfide bubbles and remove excess detritus

Coral rubble sits more densely, is less blown by current, and at 1-2 mm size visually resembles sand. Water and detritus can pass through such rubble, which is important for biofiltration.

Caring for Biofilter Substrate

Regular Overturning and Checking

• About once a month, loosen the substrate with a plastic pointer to see if there are hydrogen sulfide bubbles

• If hydrogen sulfide starts to release (especially under the reef base), remove excess detritus from the substrate, or reduce the substrate level and increase flow

• You can add "helpers" - fish and invertebrates that will stir the substrate (sea stars, sand-sifting fish)

• However, few creatures dig under the reef base, so you'll have to overturn it manually

• If the substrate starts to blacken, it's a bad sign of an anaerobic (oxygen-free) layer, where decomposition processes begin and constant hydrogen sulfide releases into the water, reducing ORP, poisoning corals and fish.

Proper "Feeding" of the Biofilter

• The biofilter substrate feeds on food remnants that fall to the bottom

• Bacteria and small invertebrates develop in it: worms, gammarus, mysids, and other microfauna

• This microfauna processes detritus and food remnants

• The biofilter consumes about 30% of what you add as food, and 70-75% is consumed directly by your animals and corals

Balance of Microfauna

• With excessive feeding, microfauna can multiply excessively

• For example, one aquarist had several thousand fire worms in a 500-liter aquarium - during feeding, the entire bottom turned red with worms

• After reducing feeding, some worms died, but then multiplied again when feeding was restored

• It's important to maintain stable feeding - microfauna multiplies based on a certain amount of food

Caution When Servicing the Biofilter

• When cleaning the substrate, some microfauna inevitably dies or is washed away

• This can lead to temporary reduction in ORP (oxidation-reduction potential)

• Never wash substrate with fresh or hot water - this will kill beneficial bacteria

• Use seawater from the same aquarium for washing

• You can perform partial water changes (10%) and use this water to wash the substrate

Don't Place Rocks Directly on the Substrate

• A rock placed on substrate creates an anaerobic zone

• There will never be ventilation under the rock, detritus will penetrate there

• All this will rot and release hydrogen sulfide

• Rocks should stand on thin legs (no more than 5 cm in diameter) or directly on the aquarium bottom

Feeding the Biofilter

The biofilter, like any living organism, needs constant and stable feeding:

Food and Organics

• Main nutrition - food remnants and organics settling to the bottom

• Stable feeding without sharp changes is important

• Overfeeding leads to excessive microfauna multiplication

• Underfeeding can lead to starvation and degradation of biofiltration

Microelements

• The biofilter consumes not only organics but also microelements

• Sponges quickly consume silicates

• Small crustaceans depend on iodine content (it stabilizes their molting, metabolism, and reproduction)

• Even if you don't have corals, 20% dosing of certain microelements like Iodine is necessary to maintain the biofilter

Feeding Stability

• Stopping the dosing of any elements disrupts the general biology of biofiltration

• By changes in ORP, you can notice the influence of microelements on the biofilter

• After starting to dose certain microelements, you may see acceleration of biofilter work and an increase or decrease in ORP

The Relationship Between Substrate and Water Flow

Substrate and flow are inextricably linked - without proper flow, substrate biofilter cannot function normally. The flow should:

• Blow through the substrate, preventing stagnant zones and anaerobic processes

• Lift detritus, preventing it from accumulating in a thick layer and rotting, releasing hydrogen sulfide

• Mix water throughout the display volume

• Carry excess detritus to the overflow, directing it to the filtration system

• Ensure degassing of harmful substances through the water surface

Proper Organization of Water Flow

For effective operation of a system with substrate, it's necessary to create three types of flow:

Wave Water Movement (horizontal)

• Created by special devices (wave boxes) like Tunze or VorTech

• Wave amplitude should be at least 3 cm from each edge (3 cm up - down, then the same from the other side)

• This movement pushes water through the reef for a distance of 15 cm

• Resembles the natural movement of water in the sea, where dirt on sand constantly moves

• Thanks to such movement, dirt doesn't settle on corals but is constantly washed away

Circular Flow (horizontal)

• Created by a return pump directed to one side along the back wall of the aquarium

• The return pipe blows in one direction at a certain speed, creating circular flow

• Works together with wave movement: the wave dragged water to one side, circular flow blew it away, then the wave dragged it to the other side

• This creates constant mixing through the interaction of two flows

Vertical Mixing (bottom to top)

• Raises bottom water upwards and top water downwards, creating a vertical circulation

• Critically important for ventilating ammonia and harmful heavy gases that concentrate in the lower water layers and caves

• Ammonia and hydrogen sulfide accumulate in substrate, especially if the layer is thick and poorly ventilated

• The forming ammonia and hydrogen sulfide can poison fish sleeping in lower water layers and caves at night

• Pumps should raise this bottom water upwards, where degassing occurs

Additional Flow Recommendations

Enhancing Surface Evaporation

• Recommended to use a fan breaking the surface film of water

• This creates ripples on the surface and enhances evaporation

• Through enhanced evaporation, harmful substances degas more quickly if necessary

• The logic is simple: lift hydrogen sulfide upwards with bottom flows, and through surface evaporation remove it from water more quickly

Degassing Harmful Substances

• Many toxic substances in the aquarium are dissolved gases

• The biofilter processes organics, but gaseous compounds are released in the process

• Example: fish feces → processing by microplankton → processing by bacteria → ultimately transformed into nitrates, etc.

• These compounds transition to gaseous form at a certain stage

• Gases need to be raised from the bottom and degassed through the water surface

Signs of Improper Flow

• Accumulation of detritus in certain places (especially under the reef)

• Appearance of black layers in the substrate

• Growth of cyanobacteria in places where dirt accumulates

• Fish developing Oodinium in the mornings (sign of ammonia and hydrogen sulfide release at night)

• "Bald spots" on corals - black dead points where dirt stayed and isn't washed away by current

Periodic Change of Flow Regimes

• Once a day, it's recommended to change pump operation modes

• This prevents the formation of stable detritus accumulation zones

• For example: "wave" mode turned on, knocked detritus to one side, switched to another mode which raised all the detritus

• The goal is to avoid places where dirt constantly accumulates

Detritus Accumulation in Substrate

An inevitable problem in aquariums with substrate is detritus accumulation:

Accumulation Spots

• Pumps drive dirt to certain places, usually under the reef or in corners

• The substrate surface may seem clean, but detritus accumulates inside

• Initially, the substrate surface is clean (top 0.5 cm), but deeper there are deposits of various rotting matter

• When a layer of detritus appears on the substrate surface, it's often followed by cyanobacteria growth

Fighting Detritus Accumulation

• Regularly siphon problem areas with a hose

• If necessary, replace part of the substrate with new clean rubble

• Change the flow organization so dirt doesn't accumulate in one place

• Use flow modes that regularly mix the substrate

Problem Zones Under the Reef

• Under the reef is usually where the worst flow occurs

• Dirt accumulates there, hydrogen sulfide and ammonia are released

• Especially dangerous at night when pumps often work in reduced mode

• Fish sleeping in these zones may suffer: in the morning they have cloudy eyes, slight signs of disease, everything passes during the day, and overnight the situation repeats

Proper Flow Through the Reef

Flow should pull water through the reef, no matter how dense and thick it is:

The Wave Should Pass Through the Reef

• Water should be pulled through the entire reef

• Minimum wave amplitude - 3 cm from each edge

• This ensures water movement of 15 cm inside the reef

Preventing Dirt Settlement on Corals

• Proper flow constantly washes dirt off corals

• In nature, dirt on sand constantly moves, not settling on corals

• With insufficient flow, dirt settles on corals, leading to "bald spots" - black dead points

Alternatives to Substrate in the Aquarium

If you're not ready to care for a biofilter substrate, there are alternatives:

Bare Bottom

• The simplest solution - completely abandon substrate

• Biofiltration needs to be provided in the SUMP (live rocks, bioceramics)

• Can use a cassette with live biosubstrate volume of 15-20 liters (depending on system volume)

• Such a cassette will start working in about a week, ORP will begin to rise

• Full launch of biofilter on such substrate will take 4-5 months

Decorative Solutions for Bare Bottom

• Sheet of white acrylic on the bottom, not reaching the front edge

• Front edge and acrylic surface sprinkled with a thin layer of sand

• When pumps blow away sand, exposing this white acrylic, it's not noticeable

• Acrylic doesn't get covered with algae, as sand periodically covers it

• Visually such a bottom looks like sand, but is much easier to maintain

Populating the Bottom with Corals

• Fast-growing corals on the bottom: zoanthids, briarium, xenia, clavularia

• Create a "flower field" effect

• Don't let them approach the reef - regularly trim edges, leaving 3 cm distance to rocks

• If these corals crawl onto rocks, they'll be difficult to remove

• Over time, they'll create a dense cover that will be covered with detritus

• Good flow is necessary to clean these corals from detritus and periodic "storms" (strong flows in different modes)

Substrate in the Sump

• Tray with sand in one of the sump compartments

• Sand layer 3-4 cm, requires overturning about once a month

• If a lot of cloudiness rises when loosening, the substrate requires more frequent cleaning

• Pay special attention to tray corners - usually the largest amount of detritus accumulates there

• Such a sand reservoir acts as an "appendix" - a storage of beneficial bacteria

• Just as the appendix contains strains of beneficial intestinal bacteria in humans, this sand container contains bacterial strains necessary for biofiltration

Starting a System with Substrate

Substrate can significantly accelerate the launch of a new system:

Using "Live" Sand

• Take colonized sand from an operating aquarium

• Pour a 3-centimeter layer of such sand

• Place the reef on thin legs (no more than 5 cm in diameter) to avoid creating anaerobic zones

• The system will start much faster than on dry rocks without substrate

Gradual Reduction of Substrate Layer

• After system launch (approximately 3-4 months), you can begin reducing the substrate layer

• Reduce the level by about 1 cm, leaving 2 cm

• Observe parameters - there should be no sharp drop in ORP or other problems

• If everything is stable, you can reduce the layer further

Preserving Removed Substrate

• Don't throw away or dry out the removed substrate

• Place it in a separate container with seawater

• Add a pump for circulation

• Such substrate can be stored for a week without problems, and with a pump - up to a month

• If necessary, you can return this substrate to the system

Conclusion

The choice between an aquarium with substrate and without it is weighing pros and cons considering your system maintenance capabilities.

Pros of substrate:

• Quick system startup

• Parameter stabilization

• Additional biofiltration

• Production of beneficial elements for corals

Cons of substrate:

• Requires regular care and overturning

• Creates risk of anaerobic zones and hydrogen sulfide release

• Can accumulate harmful substances with improper maintenance

• Necessity to organize a complex flow system

If you have the opportunity to prepare a good biofiltering cassette in advance (15-20 kg of Siporax or live rocks), you can do without substrate. In this case, after a week you'll see how ORP begins to rise, the system cleans itself, and you can gradually populate the inhabitants.

Regardless of choice, remember the importance of properly organized flow, which is a key factor for the health of the entire system