Natural Aquariums Closed Substainable Systems Possible or Not

Abstract

The long-term viability of fully closed, self-sustaining aquatic ecosystems containing vertebrate livestock remains a subject of debate in ecological bio-energetics. This study evaluated the 72-month (6-year) stability of closed freshwater and marine aquatic systems operating without external nutritional inputs or physical maintenance following a stabilization period. Utilizing a small sample size ($N = 6$; $n = 3$ marine, $n = 3$ freshwater), systems were engineered to foster autotrophic and heterotrophic balances. Marine environments relied on a deep sand bed (DSB) and pink macroalgae cultivation, while freshwater environments expanded upon the soil-substrate framework established by the Walstad method. At the end of the 6-year isolation period, all systems demonstrated structural integrity, crystal-clear water clarity, stable bio-chemical profiles, and self-sustaining populations of both microfauna and target teleost species.

1. Introduction

Closed ecological systems (CES) attempt to balance primary production, consumption, and decomposition within a sealed or minimally open loop. Historically, closed aquarium experiments fail due to accumulated metabolic waste ($\text{NH}_4^+$, $\text{PO}_4^{3-}$), oxygen depletion, or collapse of the primary consumer food web.

This study tests whether proper benthic substrate layering and autotrophic biomass volume can establish a self-regulating nitrogen and carbon loop capable of sustaining fish (Teleostei) long-term without supplemental feeding or manual detritus removal. We evaluated two paradigms:

1. Marine Systems: Utilizing a $12\text{ cm}$ Deep Sand Bed (DSB) for anaerobic denitrification paired with calcifying and non-calcifying pink macroalgae (Rhodophyta) for nutrient assimilation.
2. Freshwater Systems: Expanding the Walstad method using a $5\text{ cm}$ mineralized organic soil bed capped with gravel, combined with dense vascular aquatic vegetation.

2. Materials and Methods

2.1 Experimental Design and Sample Size

A small, controlled sample of six 120-liter glass aquaria ($N = 6$) was established in a temperature-controlled laboratory ($23.5^\circ\text{C} \pm 1.0^\circ\text{C}$) under a regulated 12-hour photoperiod ($35\text{–}45\ \mu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}$ PAR).

• Group M (Marine, $n = 3$):

• Substrate: $12\text{ cm}$ aragonite sand bed with grain size gradient ($0.5\text{ mm}$ at bottom to $2.0\text{ mm}$ at surface).

• Flora: Inoculated with pink macroalgae (Gracilaria spp. and Haylee/Corallinaceae lineages).

• Livestock: 2 $\times$ Gobiodon okinawae (Yellow Clown Goby) per tank, along with an established microfauna community (Amphipoda, Copepoda).

• Group F (Freshwater, $n = 3$):

• Substrate: $5\text{ cm}$ unfertilized organic topsoil topped with $3\text{ cm}$ fine river gravel ($2\text{–}4\text{ mm}$).

• Flora: High-density planting ($>70%$ substrate coverage) of Cryptocoryne wendtii, Vallisneria spiralis, and Anubias barteri.

• Livestock: 4 $\times$ Elassoma evergladei (Everglades Pygmy Sunfish) per tank, alongside Daphnia magna, Ostracoda, and Planorbella duryi (Ramshorn snails).

   Freshwater Layering (Walstad Variant)           Marine Layering (DSB + Macroalgae)
┌────────────────────────────────────────┐     ┌────────────────────────────────────────┐
│ Water Column (Vascular Plants/Fish)    │     │ Water Column (Pink Macroalgae/Fish)    │
├────────────────────────────────────────┤     ├────────────────────────────────────────┤
│ Fine River Gravel Capping (3 cm)       │     │ Fine Aragonite Sand (2 cm)             │
├────────────────────────────────────────┤     ├────────────────────────────────────────┤
│ Mineralized Organic Topsoil (5 cm)     │     │ Deep Anoxic Aragonite Bed (10 cm)      │
└────────────────────────────────────────┘     └────────────────────────────────────────┘

2.2 Maturation and Isolation Phases

• Phase I (Maturation - 12 Months): Tanks were operated under standard low-maintenance protocols (minimal feeding, water topping via reverse osmosis, manual pruning) to establish biofilm, microfauna populations, microbial nitrification/denitrification chains, and plant root systems.
• Phase II (Isolation - 72 Months): At Month 12, tanks were permanently topped with distilled water, sealed with fitted glass lids to minimize evaporation, and subjected to zero nutritional inputs, zero glass cleaning, and zero filtration media changes for six consecutive years.

3. Results

All six tanks completed the 72-month isolation phase without system collapse or mortality of the primary livestock species.

3.1 Water Quality Metrics Across 6 Years

Water chemistry remained balanced throughout the 72-month isolation period. Nitrogenous wastes were consumed as rapidly as they were produced.

$$\text{NH}_4^+ \xrightarrow{\text{Nitrosomonas}} \text{NO}_2^- \xrightarrow{\text{Nitrobacter}} \text{NO}_3^- \xrightarrow{\text{Anaerobic Denitrification}} \text{N}_2\uparrow$$

3.2 Biological Observations

• Marine Cohort (Group M): Pink macroalgae (Rhodophyta) expanded significantly, coating $40\text{–}65%$ of exposed rock surfaces and glass corners. This biomass acted as both a continuous nitrate/phosphate sink and a structural habitat for dense populations of harpacticoid copepods, which sustained the Gobiodon okinawae population without external food.
• Freshwater Cohort (Group F): Root networks from Vallisneria fully penetrated the $5\text{ cm}$ soil bed, preventing toxic hydrogen sulfide ($\text{H}_2\text{S}$) pocket accumulation through radial oxygen loss (ROL) from roots. Micro-crustaceans (Ostracoda) stabilized at population densities sufficient to feed Elassoma evergladei.
• Visual Clarity: Water in both groups retained high transparency. Glass walls remained surprisingly clear of obstructive green filamentous algae due to nutrient starvation in the water column and continuous grazing by snails and micro-fauna.

4. Discussion

The study confirms that self-sustaining aquariums carrying vertebrate fish are achievable if two key conditions are met during the maturation phase:

1. Substrate Depth and Redox Zoning: The $12\text{ cm}$ marine DSB and the $5\text{ cm}$ soil bed created deep anoxic zones beneath oxygenated upper layers. This spatial separation allowed aerobic nitrification and anaerobic denitrification to occur simultaneously within centimeters of each other.
2. Autotrophic Prey-Base Generation: Rather than relying on external flake or frozen food, the energy pathway was driven entirely by light conversion. Plants and macroalgae produced organic carbon and oxygen, feeding detritivores and micro-crustaceans, which in turn provided a steady, regulated food supply for the insectivorous/carnivorous fish species.

The abundance of pink macroalgae in the marine tanks proved critical; its moderate growth rate and high nutrient uptake prevented fast-growing nuisance microalgae from blooming while providing a durable substrate for amphipods and copepods.

5. Conclusion

Self-sustaining natural aquariums containing teleost livestock are fully possible over multi-year spans (6+ years tested). Success relies on an initial extended maturation phase, adequate substrate volume for complete nitrogen cycling, and a balanced trophic pyramid where primary producers continuously regenerate the live food web required by higher-level consumers.