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The conversation we hear most often from aquaculture operators runs something like this: the pilot system is working, stocking density is proving out, and now they want to double or triple production volume. The quote for an equivalent number of fibreglass or polyethylene tanks comes back at a figure that makes the expansion economics difficult to close — or they're fitting out a shed or polytunnel that wasn't engineered for the point load of permanent rigid infrastructure.
Flexible open-top tanks address a specific part of this problem. Not every part — but for operators scaling quickly, working in non-purpose-built facilities, or exporting production infrastructure to remote sites, the cost-per-litre and logistics case is hard to match with rigid alternatives.
ZolinAg round open-top tanks in a land-based aquaculture facility — circular geometry supports uniform circulation under central aeration.
How the Tank Actually Works
The ZolinAg Round Open-Top Fish Tank is a cylindrical, open-top tank made from reinforced PVC-coated fabric with HF-welded seams. The sidewall carries the load through fabric tension when filled; when empty, it deflates and folds flat. There's no rigid frame, no assembly beyond unrolling and connecting the inlet.
A few operational characteristics that matter for aquaculture specifically:
Open top. Unrestricted access for feeding, netting, aeration equipment, and stock observation. No confined-space constraints during routine husbandry.
Circular footprint. Circular geometry promotes uniform water circulation under central aeration — a radial flow pattern that sweeps waste toward the tank centre and out through a bottom drain. This is the geometry BFT operators design around, and it translates directly into the flexible format.
PVC-coated inner surface. Non-porous and smooth. Resists biofilm adhesion and is compatible with standard aquaculture disinfection protocols — dilute hydrogen peroxide, potassium permanganate, dilute hypochlorite. The surface doesn't harbour pathogens in material microstructure the way some textured rigid surfaces can.
Foldable when empty. A 20,000 L flexible tank folds to a fraction of the volume of an equivalent rigid tank. This matters for operators who clear facilities between cycles, for seasonal operations, and for anyone shipping production infrastructure to a remote site.
Where Flexible Tanks Perform Best: BFT Systems
Biofloc technology systems maintain high-density stock in tanks with minimal water exchange, relying on microbial communities to process waste. This places specific demands on tank geometry and surface characteristics — and flexible open-top tanks meet them well.
Circular geometry distributes aeration-driven circulatory flow evenly. The smooth inner surface doesn't trap biofloc aggregates in corners or surface texture. Tanks can be sized for the target production volume rather than constrained by standard rigid sizes — ZolinWorks supplies custom diameters on enquiry, which matters when you're designing a system around a specific stocking density target rather than fitting production into available tank sizes.
When a production cycle ends and tanks require full cleaning and disinfection, draining a flexible tank flat and cleaning it with a pressure washer is more practical than cleaning the interior of a fixed rigid vessel. We've had operators tell us this alone is worth the switch for their biosecurity protocol.
Hatchery Water Supply and Grow-Out Stages
Hatcheries require clean, temperature-stable holding water at specific volumes for larval tanks, holding systems, and water treatment stages. The deployment speed advantage is relevant here — a flexible tank can be filled and operational within hours of site delivery. For seasonal hatchery operations, tanks can be stored flat between seasons rather than maintained year-round.
Grow-out for land-based species — tilapia, barramundi, catfish, trout — is where the cost-per-litre case is most direct. The capital required to install flexible tanks for a given production volume is typically significantly lower than for equivalent rigid infrastructure. For operations at the proof-of-concept or early commercial stage, this changes what's financeable.
Food-safe TPU-coated fabric is available for applications requiring certified food-grade material contact. Specify at enquiry.
Central bottom aeration drives a radial circulation pattern — waste solids sweep toward the centre drain rather than accumulating in corners.
System Design: The Details That Matter
Aeration attachment. For BFT systems requiring vigorous aeration at high stocking densities, ZolinWorks can add reinforced attachment patches at specified positions for through-hull fittings or diffuser anchor points. Specify the position and fitting size at enquiry.
Inlet and outlet configuration. Standard ZolinAg tanks ship with a bottom outlet and a side inlet. For RAS or flow-through systems, specify: inlet fitting size and connection type (BSP, camlock, or other), outlet position (bottom centre for waste collection or side-wall low-point for gravity drain), overflow fitting if required, and any additional through-hull positions for sensor probes or harvest ports.
Shade and UV. Open-top tanks outdoors need shade netting over the water surface to control algae growth, reduce evaporation, and prevent temperature stress on stock in hot climates. The tank fabric has UV stabilisers in the PVC coating, but the water surface is uncovered unless shade cloth is added as a separate element.
Biosecurity between cycles. Drain the tank fully, rinse with fresh water, apply your disinfection protocol (potassium permanganate or dilute hypochlorite are standard), rinse again, and allow to dry before refilling. If storing between seasons, fold dry and keep away from direct UV exposure.
The Cost Question
We're direct about where flexible tanks compete and where they don't. For a permanent, purpose-built facility with a 20-year horizon and no relocation requirement, rigid tanks have advantages — surface finish options, structural integration into the building, and no question about long-term material fatigue.
For operators scaling an existing business, fitting out a non-purpose-built facility, exporting infrastructure to a remote grow-out site, or running seasonal operations where storage between cycles matters — the cost-per-litre and logistics case for flexible tanks is strong. We'd rather you understand the tradeoff clearly than make the wrong call for your situation.
Questions Worth Asking Any Supplier
- What fabric weight and coating type is used — PVC or TPU? Is food-grade certification available and for which standards?
- What is the seam construction method? HF-welded seams are the relevant standard for a tank that will hold water continuously under biological load.
- Are custom diameters and depths available, or are sizes fixed to a catalogue range?
- Can through-hull fittings be added at specified positions, or is the standard fitting configuration fixed?
- What disinfection protocols has the fabric been tested against?
Empty ZolinAg tanks fold flat for storage between seasons or shipping to remote grow-out sites.
Custom sizing is available for operators with non-standard requirements. Send your target volume, preferred diameter or footprint constraint, depth preference, and fitting requirements and we'll work up a configuration.