When you start this activity, the desire is to place the greatest number of fish, with the most different existing species.
Many beginner tanks have a high mortality rate, cloudy water and foul odors. In most cases, overpopulation in the aquarium considerably increases the amount of food needed and, consequently, increases the amount of waste in the water, which can lead to an increase in ammonia and nitrite, toxic compounds for fish.
There are an infinite number of possible fish combinations for each size of tank that we could recommend.
More Fish > Waste
Space – Ten 1cm tetras do not have the same body volume as a 10cm oscar. If your idea is to keep bigger fish, the amount of space becomes an important factor to consider.
Bigger Fish > More Space
Aggression level of your fish, ie, in African cichlids, the key is to add more fish and decrease the swimming space, so that no single fish has room to establish and defend its own territory.
Or a betta fish that lives in a community pond. Bettas tend to stay on top of the pond and can get aggressive if other fish are swimming close to the surface in their territory. In that case, you can choose tank mates that swim in the middle and bottom layers of the tank and that, for the most part, stay out of the way of the betta fish.
An aquarium is a living ecosystem and will change over time. Some species reproduce quickly and population booms can increase the waste load, so it may be necessary to remove fish to compensate. Healthy plants also grow over time, which reduces the waste load but reduces the available space.