4

Fish tanks for aquaponics come in a wide range of shapes, sizes, and materials, with selection being largely based on culture species. The majority of large systems use round tanks that either have a flat- or cone-bottom. Use of tangential flow will prevent dead zones when used in

round tanks (Figure 2). Cone-bottom tanks allow solids to concentrate at the bottom (in the cone) and be easily flushed from the system. Flat-bottom tanks are more widely available, but solids removal requires additional steps to ensure proper removal of organic material dispersed across the bottom of the tank. Square tanks may also require additional cleaning as solids or debris can settle in corners (Somerville et al. 2014). Sizing for fish culture tanks follows RAS principles, with a 3:1 width to height ratio being ideal for proper water movement and flow. Fish tanks are generally the highest point of the system and water flows via gravity to the solids filtration component.

Commercial-grade tanks are commonly made from strong, UV-stable materials like high-density polyethylene (HDPE) plastic or fiberglass. On a smaller scale or in areas with limited resources, intermediate bulk containers (IBC) or lined cement troughs may be utilized. Food-grade and UV resistant materials are necessary as many repurposed tanks may have held chemicals or hazardous materials, making them unsuitable fish intended for consumption.

Effective solids filtration is a key component to a well-functioning system and potentially the most important aspect as it influences the efficiency of all other processes. Solids are mostly produced from uneaten feed, fish waste, and bacteria biofilms (classified as suspended solids) (Timmons and Ebeling 2013). If waste is not removed, it can settle on plant roots (preventing uptake of nutrients), collect in areas of low water flow (resulting in poor water quality), cause the build-up of noxious gas, and clog pipes (preventing sufficient water flow) (Somerville et al. 2014).

The solids filtration utilized depends on the quality and quantity of feed entering the system, with all designs coming directly from RAS technology. The two main categories of solids filtration are sedimentation and mechanical filtration (Lennard 2012).

Sedimentation: Sedimentation refers to solids settling from the water column via gravity, which occurs in the clarifier. Clarifier, or (solids removal) designs include baffles, radial flow filters, and swirl separators (Figure 3a, b, c). Radial flow separators are most commonly used and have been shown to be more effective at removing settleable solids than a swirl filter in RAS (Davidson and Summerfelt 2005). Baffle and swirl clarifiers are similar in solids removal efficiency (Danaher et al. 2013). Recommendations for construction material follow that of fish tanks mentioned above.