I also think it depends on the species of fish, and also the other tank dimensions. If the tank is 8ft long, then it doens't have to be as wide, there is swimming space, do a 3 point turn, swim back. Thats the theory I'm using with my 'new' 1.8m tanks and my plecos anyway.
A gourami naturally comes from fairly cramped conditions I believe, Paddy fields and small streams. They aren't really an open water fish, so they can handle smaller dimensions, as long as your total tank volume is big enough.
Something faster swimming like a bala shark on the other hand I would suggest you want twice the fishes length for it to turn around easlily, so a 30cm bala should probably have 60cm depth and probably height as well.
Depth is also debatable. Many fish could live quite well in water not much deeper than their body, gourami and plecos for instance. The eels in by back garden seem quite happy in 10-15cm of water, and they are 50-100cm long. When they swim up the rapids they are actually 1/2 out of the water.
But generally larger tanks are usually at least 40-50cm deep, plenty of depth for most fish.
Ian