You can have 15 full grown rasboras in a 29-gallon tank, provided you use modern filtration and stock no other fish. However, if you are keeping them with other species of fish, we recommend you stock at least 8 harlequin rasboras to accommodate their schooling behavior and keep them happy.
Although there are many species of rasbora, the harlequin rasbora is the most commonly kept in 29-gallon community aquariums. For example, the smaller species of rasbora in the genus Boraras are more suited to nano tanks, and the few larger species of rasbora require larger tanks than 29 gallons.
The Inch Per Gallon Rule For Rasboras In A 29 Gallon Tank
A widely quoted rule for stocking fish tanks states that one should allow one inch of fish for every gallon of water. You may sometimes see it quoted as an inch per two gallons of water.
This rule is intended to allow a certain amount of water per inch of fish to absorb the waste that the fish produce. However, it is not always accurate, as larger-bodied fish create more waste and need a greater volume of water.
Because of their relatively slim, slender shape, rasboras such as harlequin rasboras hew to this inch per gallon rule well, and it is a pretty accurate measure of how many you can stock in a tank.
All other things being equal, you can, therefore, stock around 15 harlequin rasboras, with their adult size of two inches, in a 29-gallon tank. Remember that you should always consider the full adult size of the fish when determining stocking density.
Why A 29-Gallon Tank Does Not Hold 29 Gallons
You should bear in mind, though, that several factors distort the inch-per-gallon rule. A 29-gallon tank is liable to hold gravel (or sand), rocks, and plants in addition to water, meaning its actual capacity is not 29 gallons of water.
Once you have accounted for the substrate and plants, you will have around ten to 15 percent less water than the stated capacity. A 29-gallon tank will, therefore, hold around 25 to 26 gallons of water.
Using the inch per gallon rule, one should be able to keep around 12 to 13 harlequin rasboras (or similar species such as lambchop rasboras or glowlight rasboras). This figure is the total number of fish you could keep in such a tank, which would be a single-species aquarium.
Why You Can Keep More Rasboras In A 29-Gallon Tank
From the inch per gallon rule and harlequin rasboras adult size of two inches, you would be limited to around a dozen of the fish in a 29-gallon planted tank with no other fish in it.
However, plants in an aquarium help to soak up fish waste, as they use waste nitrogen compounds as fertilizer. You can, therefore, stock more generously in a planted aquarium.
Moreover, the inch per gallon rule was formulated before the advent of modern filtration systems. Whether using a canister filter or a hang-on-back, we assume you are using adequate filtration in your aquarium to deal with fish waste.
Adequate filtration systems in a tank mean you can stock far more fish. If you are using enough filtration, you can keep around 30 harlequin rasboras in a 29-gallon tank, with no other fish being present, i. e. a single-species tank.
If, on the other hand, you have other fish, such as corydoras or kuhli loaches on the bottom, and a gourami or two, you will want to keep the number of rasboras to around 12. This caveat applies if you stock lampeye rasboras or galaxy rasboras (also known as celestial pearl danios).
If you keep a smaller species of rasbora, such as the chili rasbora that only gets to around 0.7 inches when fully grown, you can keep two of them per gallon, as they have a very low bioload. You could, therefore, stock around 50 of these fish in a 29-gallon tank if you kept nothing else.
Of course, aquarists don’t usually keep tiny species of rasbora, such as the chili rasbora, in tanks as large as 29 gallons. So, although we have provided you with the information on how many you could keep, you would probably want to keep them in a smaller tank with small, peaceful tankmates.
Rasboras Are Active Swimmers And Schooling Fish
One also has to consider the behavioral traits of the specific fish species when determining stocking density. Fish need room to swim, and actively-swimming fish such as rasboras require more space than sluggish fish.
Although one could, theoretically, keep 30 harlequin rasboras in a 29-gallon tank, they would be rather cramped and unable to swim about normally due to their active schooling behavior. However, one does have to keep multiples of schooling fish such as rasboras to prevent them from becoming stressed.
Having too few individuals of a schooling species makes them stressed and jittery, and they will tend to hide and have a shortened lifespan. To keep your rasboras healthy, keep them in groups of at least seven.
Keep your schooling fish such as rasboras in odd numbers of seven, nine, eleven, or more to ensure an uneven ratio of males to females that creates interest and competition in the school, holding them together. If you do not do this, they will pair off in different corners of the tank.
Larger schools make for a stunning display, provided you give them enough space to swim freely.
Tank Mates For Harlequin Rasboras
Although the harlequin rasbora tends to keep to itself in nature, you can keep them in community tanks provided you supply plenty of hiding places and choose peaceful tank mates that will not harass or attack them.
Good tank mates for harlequin rasboras include bettas, neon tetras, cardinal tetras, dwarf gouramis, honey gouramis, kuhli loaches, cory catfish, danios, and other small rasboras. Your rasboras will be peaceful fish that will not cause trouble with other fish through nipping or squabbling.
Conclusion
Now you know the rules of thumb for stocking tanks with rasboras and what factors affect how many you can keep. You will be able to determine how many you can keep in a 29-gallon tank with the other fish you have in there.