There are quite a few reasons, but some common ones are that some fish need room to grow, some fish need room to swim around, and some fish need room to claim a proper territory.
It depends on the fish tank. They range from less than 2 gallons to over 300 gallons.
The unit typically used to measure the amount of water in a fish tank is liters (L) or gallons (gal).
A certain amount of oxygen dissolves in water (thus enabling fish to breathe).
The hydrosphere contains water. There are a certain amount of other things, such as fish, that are found in the water. But it's mostly water.
the amount of fish are always changing and there is no certain answer.
Some aquarium salt is good for fish, the recommended amount is 1 tablespoon for every 5 gallons, however some aquatic plants and scaleless fish are very sensitive to salt and it will kill them. Cory Catfish are one of those fish.
They're measured in gallons in the US, and liters everywhere else. This holds true no matter how big the tank is--the New England Aquarium in Boston has a tank they describe as holding one million gallons of water.
2.25
These fish like warm water
Approximately 43 gallons
You need about one gallon for every inch of fish you have
That would be appoximately 45 gallons, but its probably slightly less than that due to the thickness of the glass and the trim(if they is trim.