[header] How to know how many fish you can place in a fish tank [title] Measure your tank. [step] The standard measurement is liters. You should be able to figure out how big your tank is when you initially purchase it. 
But if you don't know (or don't remember) the size, you can figure it out on your own mathematically. [substeps] To find out how many liters your tank is measure the length, width, and height of the tank in centimeters and multiply them together, then divide by 1000.