I have heard a huge range in answers to this one from people online. Even the books that I have read vary a little bit dependent on the author. But not as drastic as the online opinions. I have heard more often than I thought I would “10 gallons to every 1 inch of fish.” That seems to be a bit much even for marine fish.
Everything I have read by people that seem to know what they are talking about leads me to the general rule of thumb of 1 gallon for every 1 inch of fish for freshwater fish and about 3 gallons for saltwater fish. If anyone has other opinions feel free to share them in the comments below.
I was recently thumbing through a book I purchased years ago, “Freshwater Aquariums for Dummies 2nd Edition” from 2006 by Maddy and Mic Hargrove. Good book for people first getting into freshwater aquariums. Notice no affiliate links anywhere. It really is just a good book for beginners. They tackle the question of “How many fish you can I put in a tank?” with a very useful chart that seems reasonable.
The first column is the size of the tank by length and width. That then calculates the surface area which is the second column. The last two columns are based off of the first two columns which leads to how many inches of fish for both freshwater and saltwater (marine) that can be supported in that tank.
|Tank ( Length x Width) inches||Surface Area inches||Freshwater Fish inches||Saltwater / Marine Fish inches|
|20 x 10||200||16||4|
|24 x 12||288||24||6|
|30 x 12||360||30||7|
|36 x 12||432||36||9|
|48 x 12||576||48||12|
|48 x 18||864||72||18|
|72 x 18||1296||108||27|
|72 x 24||1728||144||36|