One of the most common questions about fish, especially from hobbyists just starting out, is how many fish can I keep in a tank? The answer is, "It depends." It depends on several things.

How big do the fish eventually get? Most LFS's sell juvenile fish. Is your tank 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 1 oscar is not equal to 1 neon tetra. And 1 inch of neon does not equal 1 inch of oscar. It also really depends on whether you have live plants in the tank, and how much you feed your fish.

To give an example – a 20 gallon high started from scratch, using 60 percent water from an existing tank and 40 percent new conditioned tap water. The filter for this aquarium is a typical "waterfall" filter that hangs on the side of the tank, which was taken off a larger tank that was well established. The tank has 0 ammonia, 0 nitrite and just a trace of nitrate. The tank has many live plants and a very good new lighting system. The tank works because:

1) With the water, filter and plants from a good existing tank the Nitrogen Cycle was already pretty much established.

2) The live 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 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!

Photo by David Lass

Photo by David Lass

Now for the fish in the tank:
8 medium angelfish, about silver dollar body size, to be moved to a larger tank soon.
2 giant danios.
2 platies.
2 Columbian red fin tetras.
3 cory cats.
3 clown plecos

By any of the "rules" this tank is way overcrowded – and yet it works. The fish are fine and thriving, the water condition is ideal, the plants growing well. The keys are that the fish were all healthy and not stressed to start with, they are not being fed too much, there are lots and lots of live plants in the tank, and the tank gets regular weekly water changes. Rather than going by hard and fast rules for how many fish can be kept in a tank, it is much more important to understand what things determine how many fish you can keep in a tank.