1. Abstract

Tilapia (Oreochromis) is the world’s second-largest farmed fish species, with global production reaching 8.1 million tonnes in 2025. In Recirculating Aquaculture Systems (RAS), the choice of pond type directly influences tilapia’s growth rate, feed conversion ratio, survival rate and ultimate economic returns. Based on a 120-day comparative trial of tilapia reared in RAS systems, this paper systematically compares the performance differences — in terms of weight gain rate, growth curves, feed conversion ratio, water quality parameters and economic returns — among six pond types: circular ponds, octagonal ponds, D-shaped ponds, square-cornered round-bottomed ponds, raceways and traditional earthen ponds. It aims to provide tilapia farmers with a scientific basis for selecting the most suitable pond type.

2. Experimental Design and Methods

Experimental species: Nile tilapia (Oreochromis niloticus), GIFT strain, initial body weight 15±2 g.

Experimental period: 120 days (4 months).

Experimental setup: Three replicates were set up for each of the six pond types, with an initial stocking density of 200 fish/m³ per pond.

Table 1: Parameters for different types of fish ponds

All RAS tank types share the same water treatment system (rotary drum microfiltration unit + MBBR biological filter + UV disinfection + pure oxygen cone), with a constant water temperature of 28±1℃.

3. Experimental Results

3.1 Harvest Weight and Weight Gain Rate

Figure 1: Comparison of harvest weights of tilapia in different pond types after 120 days

Table 2: Growth Performance and Rankings of Tilapia in Different Pond Types

3.2 Comparison of Growth Curves

Figure 2: Growth curves for six types of tilapia over 120 days

The growth curves clearly show that, from day 30 onwards, the tilapia in the circular and octagonal ponds began to outperform those in other pond types, with the gap continuing to widen over time. By day 120, the difference in body weight between the circular ponds and the earthen ponds had reached 190 g.

Key findings on growth curves in aquaculture:

(1). Days 0-30: Little difference between pond types (15g → 80-100g); the fish are still small, so environmental differences have a limited impact

(2). Days 30-60: Round and octagonal ponds begin to take the lead; the waste removal advantages of the rotating water flow become apparent

(3). Days 60-90: The gap widened significantly, with round tanks at 260g versus earthen ponds at 140g — a difference of 85 per cent

(4). Days 90-120: Round tanks at 385g versus earthen ponds at 195g — a difference of 97 per cent; fish in round tanks were approaching market size

3.3 Comprehensive Comparison of FCR, Survival Rate and SGR

Figure 3: Comparison of multiple indicators: FCR, survival rate and SGR

Table 3: Multi-indicator data for FCR, survival rate, SGR, feed wastage rate and a comprehensive comparison across different pond types

The FCR for circular ponds is just 1.10, which is 40 per cent lower than the 1.85 recorded for earthen ponds. Based on an average tilapia feed price of US$0.90 per kilogram, circular ponds can save US$670 in feed costs for every 1,000 kg of tilapia produced.

3.4 Comparison of Water Quality Parameters

Figure 4: Heatmap of water quality parameters for six pond types (green indicates the optimal value)

Table 4: Detailed water quality parameters for six pond types

The circular pond performed best across all water quality parameters: it had the highest dissolved oxygen level (7.2 mg/L), the lowest ammonia-nitrogen level (0.08 mg/L) and the highest removal rate for suspended solids (92 per cent).

4. Discussion

4.1 Comparison of economic benefits

Figure 5: Comparison of annual production and net profit across different pond types (tilapia RAS)

Figure 6: Comparison of 10-year cumulative return on investment curves

Table 5: Details of the cumulative return on investment over ten years for each pool type

Round ponds generate an annual net profit of 120-140 yuan per square metre, which is 8–14 times that of traditional earthen ponds. Over a 10-year period, the cumulative net profit for round ponds can reach 1,100–1,300 yuan per square metre, which is 9-16 times that of earthen ponds.

4.2 Comprehensive Scores and Rankings for the Six Pond Types

Table 6: Detailed parameters and scores for the six pool types

Overall Score: Round ponds ranked first with a score of 9.50, followed closely by octagonal ponds with a score of 9.00. Both are preferred pond types for tilapia RAS farming.

5. Conclusions and Recommendations for Selection

Table 7: Application scenarios and reasons for selecting different pool types

This 120-day trial confirms circular tanks as the optimal choice for tilapia RAS farming. Octagonal tanks are the best alternative for space-constrained projects. Selecting the right tank type is the first and most critical step toward maximizing tilapia RAS returns.

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