The biological load on the tank should be incremented slowly to give bacteria colonies a chance to expand to meet the additional processing needs.
The rule of thumb is meant for novices. Accomplished aquarists can keep a much greater density of fish. That’s because there are a number of factors that determine the allowable stocking level in the tank. To wit:
Temperature. The cooler the water, the more oxygen it can hold.
Water changes. The more often you change the water, the less toxins and waste in the tank, the more fish it supports.
Feeding. The more you feed the fish, the more waste they produce.
Plants. Plants process waste and consume carbon dioxide during the day. Their use of oxygen at night is negligible.
Body mass. Older guppies have greater body mass per inch.
Surface to volume ratio. Shallow tanks support more fish per gallon than tall aquariums. The surface of the aquarium is where oxygen is absorbed into water and carbon dioxide dispersed.
Agitation. The more the water circulates in the aquarium, the more oxygen is added and carbon dioxide lost.
Chemistry of the water. This affects the amount of oxygen that the water can hold. It also affects the metabolism of fish.
You can understand why the hobby veterans are more adept at keeping more fish per gallon. The factors affecting the aquarium’s ability to sustain life are complex and inter-related. The best plan is to follow the rule of thumb until you have become experienced at juggling the factors involved.
Here’s the rule of thumb in various formulations:
1 inch of fish per gallon.
1 cm of fish per 1.8 cubic cm.
1 cm of fish per 1 ¾ liters.
1 inch of fish per 12 square inches of surface area.
The average length of a guppy is 1.5 inches or 3.8 cm. So a ten gallon tank (38 liters) can support 6 or 7 fish according to the rule of thumb.
The guppy breeder can stock the tank with a greater number of guppies if the oxygen level of the tank is raised with additional aeration. Some breeders, for example, use two box filters in the tank and turn up the air until the water in the aquarium “boils.” However, if the filtration system fails, the tank can very quickly foul. Another factor to consider is that fish become stressed when they are crowded, and bullying often occurs.