Proper stocking in a planted aquarium depends on filtration, plant density, and maintenance rather than a strict inch-per-gallon rule. A healthy, established planted tank can support higher stocking due to plants absorbing nitrates, typically aiming for nitrate levels below 40 ppm. Ideal setups prioritize smaller fish for swimming space.
The popular rule of thumb, 1 inch of livestock per gallon, is usually not helpful.
The number of fish and other aquatic animals that can safely inhabit a tank of given size depends on three main variables.
- #1 The bio load or waste load that a tank can handle
- When aquatic animals eat they also produce waste which releases toxic compounds
- Thankfully, the nitrogen cycle provided by beneficial bacteria colonies within the tank ecosystem system, and aquarium plants (aka biological filtration) can help break down and consume those harmful waste compounds, purifying the water and making it safe for the livestock
- Biological filtration capacity will grow to meet the needs of a larger bioload (managing waste load) within reasonable limits
- Grow more beneficial bacteria
- These bacteria that break down ammonia and nitrite live on different surfaces in your aquarium, like substrate, walls, and especially the filter
- Ensure adequate filtration for the tank size, with good filter media for growing bacteria colonies
- More beneficial bacteria allow more fish, shrimp, snails ...
- Grow live aquarium plants
- Plants absorb nitrogen waste compounds as food
- More plants, especially fast growing stem plants or floating plants, means the aquarium can handle more livestock
- With more livestock, producing more waste than the bacteria and plants can handle, then water changes become necessary
- Water quality for pH, ammonia, nitrite, nitrate, and GH and KH should be tested regularly (eg., weekly)
- Nitrate level can be a good indicator of the need for a water change
- While bacteria break down ammonia and nitrite, they don't break down nitrate, so it builds up
- Plants feed on nitrate, but if they cannot consume all that's produced, then nitrate builds up
- Nitrate in the water column is not as toxic as ammonia or nitrite, but it can still be toxic for aquatic animals at higher levels
- Many aquarists consider nitrate levels greater than 40 ppm as an indication that a water change is needed
- Generally a 25% water change each week is sufficient, more fish demands more frequent water changes
- #2 The need for adequate swimming space can limit the number of fish in a tank
- Things to consider about the fish in the tank
- Maximum adult size
- Activity level of the fish
- Preferred zone within the tank, substrate, low level, mid level, upper level, hiding in plants, etc
- The number of fish that a tank size can handle depends on nature of the species kept
- #3 Aggression tendency of a species can also limit the number of fish in a given tank size
Remember that an aquarium is a living ecosystem and it depends on consistency and balance for good health.