Is the “One Inch of Fish per One Gallon of Water” Rule: Part 2

To see recommended ways aquarists compute tank requirements for computing fish stocking levels, see Part 1 in this article series. Fish stocking levels are determined by a variety of factors. This article is dedicated to how maintaining proper aquarium water parameters, filtration, and fish behaviors affect stocking levels also.

Proper water parameters are one of the most important – if not the most important - requirement to help ensure the health of your aquatic life. Water parameters include temperature, specific gravity (for saltwater tanks), pH, dKH parameters, in addition to supplements added to the water. Regular water changes are also necessary, as is testing and making necessary adjustments. Filtration, water flow, and lighting should match the care levels of each species. Observation for disease is also an on-going function.

A plan to eliminate detritus, food waste, and biological waste should also be addressed through filtration equipment, aquatic life cleanup crews, or both. Filtration systems can include mechanical, biological, chemical, or a combination. Adequate filtration must be taken into account when determining stocking levels. The higher the bioload, the greater the filtration requirement. Of note, full bodied fish will create a higher bioload than slender fish. How much food your fish requires can also add to the bioload. Herbivores, for example, must consume large amounts of food in order to get the nutrients it needs.

Managing the nitrate level in your aquarium is important. Biological colonies (and plants when present) convert ammonia and nitrites into nitrates. For fish-only and FOWLR marine aquariums, the nitrate levels should be lower than 30 parts per million (ppm), and 1.0 ppm for reef tanks. Nitrate levels for freshwater tanks should be below 20 ppm.

When housing living animals like fish, it is beneficial to study their behaviors, as some of those behaviors can have an impact on stocking levels.

• Is the fish kept singly, or is it housed in groups, schools, or shoals? Larger groups of fish strictly because of their mass require more space. Full bodied and taller fish take up more space than thinner and shorter ones.
• Territorial fish need plenty of areas they can claim as their own, in order to reduce their aggression and reduce stress among other inhabitants.
• Aggressive fish like African Cichlids may actually need to be restrained in terms of space to keep aggression in check, reducing the territory it has to defend.
• Activity level plays a part. Active fish require more swimming area and will expel more ammonia through their gills, adding to the bioload. By contrast, more sedentary species such as ambush predators require less swimming area.
• Aquarists should research what area of the water column fish like to swim. Fish selection should be conducted in a way to avoid potential problems.
• Temperament of the specific species housed comes into play in determining fish stocking levels, as well as taking into account whether your fish are peaceful, semi-aggressive, or aggressive.

Aquarists should be sure to follow information from suppliers like LiveAquaria® on aquarium setup, acclimation procedures, quarantining, and stocking schedules. It is recommended to stock no more than twenty-five percent of the fish at the same time, so water parameters stay in check, and also so that the established bacterial colonies are not overwhelmed. Introducing fish gradually allows them time to grow to meet the needs of the increased bioload being produced. It is important to ensure the filtration is upgraded if needed in order to accommodate the new additions. Aquarists who breed aquatic stock should take steps to remove fish as a counterbalance when there are new fry in the aquarium to keep the aquarium’s parameters stable. Close observation after additions or removals can help to identify any problems.

Unfortunately, there is no stocking level guideline that fits every situation. The answer lies in researching fish species to be housed, understanding fish behavior and needs, optimizing surface area, adding fish and monitoring, testing, and making adjustments, as necessary. The end goal should be to keep all aquarium inhabitants as stress-free and healthy as possible and perform the necessary tasks to keep aquatic life as healthy and stress free as possible.