Rushing water causes a lot of drag.
When you set up a tank, you of course build it according to the species you have - if they have short fins and flattish thin bodies - chances are they're a high speed stream fish - for a great example look at how the Hillstream loach has modified itself:
The hillstream has flattened it's entire structure, and spread it's fins SIDEWAYS.
This means it can create controllable suction using its underside and flattened fins, keeping water OUT from under it, and the water flows neatly OVER it - keeping it firmly fixed to the rock it wants.
It also has a relatively small gillspace, and slow breathing - fast water means more oxygen.
However - if you look at the veiltail guppy, I would dare to suggest they would be best in slower waters:
They have reasonably large gills, fast breathing, as slow water is low oxygen.
The long fins would cause drag, meaning the fish would have to work harder to get anywhere, or even stay in one place - leading to possible starvation, exhaustion and death.
The downside of fast biotope fish is that sometimes it's hard to cater to their needs - not enough flow and oxygen can push their metabolisms over due to buildup of carbon dioxide they would be unable to expell.
If there isn't enough flow, they become depressed and listless.
However, an angled flow, or angled spraybar can create a fast "wall" of water down one side - good for fast animals to play with, but maintaining a relatively slow flow throughout hte rest of the tank!

Hope that helps.