Have you ever noticed that species profiles written about marine fish almost always include a recommended minimum tank size for the species? Have you also noticed that tank-size recommendations for the same species can vary wildly from one author, dealer, or fellow hobbyist to another? How can that be? The fact is, determining the appropriate tank size for any given species is, to a certain extent, more art than science and not nearly as clear-cut as it might seem.
Obviously, the chosen tank must accommodate the growth potential of the species you plan to buy. In other words, the fish—at maximum size—must have sufficient room to swim and maneuver easily throughout the tank as well as turn around with no difficulty at either end. But that’s just one factor to consider. Here are six additional tips that will help you master the art of tank-size selection:
#1: Ignore the inch-per-gallon rule!
Right now you may be thinking to yourself, “No problem! I got this! I’ll just break out the old ‘one-inch-of-fish-per-gallon-of-water rule’ I learned back when I kept freshwater guppies and calculate the exact tank size I’ll need. Piece of cake!” Well, not so fast!
That old inch-per-gallon rule—as well as any variation upon it—is absolutely useless for determining the appropriate tank size for a species. The reason being, such rules completely ignore the overall mass of the fish. According to the inch-per-gallon rule, if I have a 20-gallon tank, I could keep either a queen angelfish (Holacanthus ciliaris) or six chalk bass (Serranus tortugarum) with a few gallons to spare. The latter scenario is almost workable while the former is utterly absurd.
Mass matters because it correlates directly with the species rate of food consumption and subsequent waste elimination. The greater the biomass, the more water volume you need to dilute those dissolved pollutants.
#2: Factor in tank configuration
Quick: is a 100-gallon tank large enough or too small to house a bird wrasse (Gomphosus varius)? Actually, either answer can be correct depending on the configuration of the tank. G. varius is a highly energetic, fast-moving species that requires lots of open swimming space. On the one hand, a lower, rectangular or racetrack-style 100-gallon tank should be able to meet these needs (though just barely). But on the other hand, I would not recommend keeping a bird wrasse in a taller, hexagonal or cylindrical tank that holds the exact same volume of water. The much narrower footprint of such tanks would be too confining for this species.
#3: Evaluate aquascaping
In addition to the configuration of the tank, consider how the aquascaping of the aquarium will affect the tank’s suitability for a given species. To illustrate this point, let’s revisit our example of the bird wrasse in a 100-gallon, rectangular tank. But this time, let’s add a stack of live rock that fills up two-thirds of the tank. Is that 100-gallon still suitable minimum housing for our explosively energetic bird wrasse? Well, not so much. It’s great to have all that live rock, but now our wrasse lacks sufficient swimming room and may no longer be a suitable option for this particular system.
#4: Survey the stocking density
Now, let’s say you have your heart set on getting a harlequin tuskfish (Choerodon fasciatus) for your established 75-gallon FOWLR (fish-only-with-live-rock) tank. You’ve done your homework on this species, and several reputable sources have assured you that 75 gallons should be sufficient. Should you go ahead with the purchase?
Before committing, and in addition to considering the previous points discussed, you’ll need to evaluate whether your 75-gallon can handle the additional bioload. If your tank is already heavily stocked, one more specimen could be one too many for your biofilter—even though the tank is technically big enough for the tuskfish by itself. In this case, if you really want to keep your current lineup and introduce a tuskfish, your best option is to upsize the system.
#5: Consider the species’ swimming habits
As I’ve already alluded in the bird wrasse example, a species’ energy level and swimming habits can have a significant impact on the size of tank that is appropriate for it. For example, a relatively sedentary bottom dweller, such as the striated frogfish (Antennarius striatus), which reaches a maximum length of between 8.5 inches and has a rather robust body to boot, can be kept in a tank as small as 30 to 40 gallons provided appropriate steps are taken to maintain good water quality. In contrast, the similarly sized but much more energetic and active Atlantic blue tang (Acanthurus coeruleus) would need an aquarium at least twice that size.
#6: Assess aggression levels
Last but certainly not least, highly territorial species are often best kept in tanks much larger than their maximum size might suggest—if you want to keep any other species in the same aquarium with them, that is. Many of the dottybacks and damsels are good examples of this. In smaller systems, they’ll oftentimes claim the entire tank for their territory and make life absolutely miserable for any other fish. On the other hand, in much larger systems with ample hiding places and room for multiple territories, they may prove to be less of a terror to their tankmates (emphasis on may).
How do you decide?
Alright, all you experienced salties out there, this list is by no means exhaustive, and I’m sure you’ve got some good tips for choosing the right tank size that you’ve picked up over the years. Please feel free to share them with us in the comments section below.