Water exchange in relation to amount of fish
The degree of re-use does not, however, give enough information about the performance of the system and is not sufficient to describe a recirculation system properly. Neither does it take into account the amount of fish in the system. This can be illustrated in the following way: there are few problems in having only one fish in a large re-use system, compared to having a high density of fish in the same system. The degree of re-use for the systems can, however, be equal. To describe the re-use system it will also be necessary to know the amount of new water added per kg fish (litres new water per kg fish).
A re-use system with a total tank volume of 100 m3 has a total circulating water flow of 2000 l/min. The re-use degree is 95%, meaning that the amount of added new water is 100 l/min. The fish density is in case 1, 10 kg/m3 and in case 2, 100 kg/m3. This represents a total amount of fish of respectively, 1000 kg and 10 000 kg, so the amount of new water in these cases is 0.1 l/(min kg fish) and 0.01 l/(min kg fish).
Even with this information it is difficult to compare re-use systems, because factors such as species, size and growth rate will have effects. For easily evaluating a separate re-use system, the growth rate of the fish is the best indicator. If the growth rate is optimal it can either be compared to growth tables or to growth in a flow-through system.