One of the most common questions about tropical fish, especially from hobbyists just starting out, is how many fish can I keep in an aquarium? The answer is, “It depends.” It depends on several things.
How big do the fish eventually get? Most local fish stores sell juvenile tropical fish. Is your fish aquarium completely cycled? This means that the “good” bacteria are there to convert fish wastes/ammonia into nitrite, and others to convert nitrite into nitrate.
The answer also involves the fact that one oscar is not equal to one neon tetra. And 1 inch of neon does not equal 1 inch of oscar. It also really depends on whether you have live aquatic plants in the aquarium, and how much you feed your tropical fish.
To give an example – a 20 gallon high started from scratch, using 60 percent water from an existing aquarium and 40 percent new conditioned tap water. The filter for this aquarium is a typical “waterfall” filter that hangs on the side of the aquarium, which was taken off a larger aquarium that was well-established. The aquarium has 0 ammonia, 0 nitrite and just a trace of nitrate. There are many live aquatic plants and a very good new lighting system. The aquarium works because:
1) With the water, filter and aquatic plants from a good existing aquarium, the Nitrogen Cycle was already pretty much established.
2) The live aquatic plants – a large Amazon sword, a big bunch of Rotala indica, a whole bunch of crypts, and a large water lettuce floating on top that needed trimming every couple of days or so – this also drives the Nitrogen Cycle.
3) The tropical fish get fed very small amounts two or three times a day – one day a week they do not get fed at all.
4) A 25 percent water change every week – every week!
Now for the fish in the aquarium:
8 medium angelfish, about silver dollar body size, to be moved to a larger aquarium soon.
2 giant danios.
2 Colombian red fin tetras.
3 cory cats.
3 clown plecos
By any of the “rules” this aquarium is way overcrowded – and yet it works. The tropical fish are fine and thriving, the water conditions are ideal, and the aquatic plants are growing well. The keys are that the tropical fish were all healthy and not stressed to start with, they are not being fed too much, there are lots and lots of live aquatic plants in the fish aquarium, and the aquarium gets regular weekly water changes. Rather than going by hard and fast rules for how many tropical fish can be kept in a fish aquarium, it is much more important to understand what things determine how many fish you can keep in an aquarium.