# Rearing Conditions and Automated Feed Distribution Systems for Zebrafish (_Danio rerio_)
## 3 Designed Culture Systems
These systems are aimed at supporting adequately embryonic and/or larval fish-raising under controlled experimental conditions, while a unique flow-through solution moves to all the raised individuals. Most of the traditional and common raising practices are, in fact, still performed manually, which imply time-consuming operations, human errors, and limited reproducibility [37]. A microfluidic system for embryonic and larval culture, where most tasks are performed automatically without external events that can disturb embryo development, has been developed [37]. Such a device allows one to transport, immobilize for imaging, continuously deliver reagents and drugs while real-time observations are performed, and retrieve zebrafish embryos post-analysis in a more efficient way compared with conventional static cultures [37]. Despite zebrafish larvae showing a more complex shape and active swimming compared with immobilized embryos, which makes the fabrication of suitable systems more difficult, microfluidic devices have also been made for this stage. Examples of microfluidic systems for raising of embryonic and larval zebrafish developed by research groups are given in Table 2. More detailed information of the microfluidic systems can be found in the review of [PERSON] and [PERSON] [38]. The next design, called recirculating system, is the most commonly used in laboratories and gives investigators the ability to control water quality and even eliminate undesired factors, such as over-exceeded ions and toxic elements. Nearly all the culture systems commercially available for zebrafish are the recirculating type and most of them have been developed as an open-formula to satisfy specific needs of the laboratories. The general scheme of a recirculating system includes the collection of tap water, previously subject to filtration, in a common filter unit which pumps water into a large reservoir (header tank). From there, the water is distributed from the top by gravity flow into each row and into each single tank where the flow rate can be adjusted according to the size, number, and age of the fish. Then, the water returns to the common filter unit for recirculation. The water source can be treated using a variety of filter systems such as charcoal filters, reverse osmosis, or ion-exchange columns, and specific ions can be added. Therefore, the main advantage of the recirculation systems includes the possibility of providing high-quality water and a high exchange rate in the tanks. Moreover, synthetic sea salt can be dissolved to achieve appropriate conductivity. However, highly purified water requires knowledge of culture engineering technologies, water management, and maintenance costs. The purification of recirculating water can be done by different mechanisms, such as the use of mechanical filters and clarifiers in combination with biological filters, ultraviolet sterilization, protein skimming, and ozonation (or combinations thereof). However, all these systems, particularly the biological filtration, often require an initial adjustment period, whereby appropriate levels of autotrophic nitriifying bacterial populations need to be established. If water parameters, such as alkalinity, hardness, and pH are not monitored and corrected as needed, filter efficiencies will fluctuate over time. Examples of recirculating systems are the open-design solutions proposed by [PERSON] et al. [39], [PERSON] et al. [40], and [PERSON] and [PERSON] [41; 42].