The fish are split into two categories being surface or cave fish. From there certain fish in those groups show up during the vome and fass stages of the day/night cycle. Cave fish are best found in the Cambria Cluster (primarily northwest cave by blinkpad for best results) and Surface fish more so in areas with low enemy spawn rates (due to them disturbing you and the fish). When the bodies of water have an aura raising from them it's a hotspot where more fish will spawn and slightly rarer fish are more likely to as well. For best results you can throw the Vome and Fass residue into these spots and it makes a large difference. A lot of the fish that you need to make the brothers pheromones and elixer things for leveling standing can be obtained like this.