Tank Width And Fish Length

[meguro]

What is a good width for larger fish, say somewhere in the size 8-10 inches. Would 14 inches suffice or is that too restrictive for when the fish wants to turn and stuff? Is there a recommended guideline for width, say X inches more than the length of the fish?

The type of fish doesn't matter right? I mean the rule would apply to geophagus, severum, green terror etc. the same?

For clarity the dimension I mean is the one in bold - 60"x14"x20" (LxWxH).

 

[Wills]

I always go for tank length = 6x fish length and tank width = 2x fish length. Im desperate looking for a 4x2x2 at the moment but cant find any non marine ones

 

[Davo86]

For fish maxing around 10" I would be looking at a 18" wide tank,