Aquatic system engineering is an important factor when designing a new fish holding system or renovating an existing system. Indoor recirculating aquatic systems may be used for various operations, some of which may include: the quarantine of new animals, isolation for ill fish, aquaculture, research, or as educational displays. Professional engineers generally design large or high-density systems using a massbalance approach. However, smaller systems are typically designed or renovated by their immediate owners, which may include aquaculturalists, aquarists, biologists, zoologists, or professors. In many instances trial and error is used to size the equipment, which can get very expensive and take up valuable time. Undersized or oversized equipment wastes electricity and possibly reduces the life of the equipment. These limitations can be avoided by using the practical guidelines given here and taking into consideration a few simple design factors. Proper design of these systems can be accomplished by much quicker methods than a full-scale mass-balance approach and will typically work for low-density systems.