water flow and fish stress question

[venzi]

So when considering water flow (particularly too much flow to induce stress in fish), what specifically causes the stress? 

1. Is it by virtue of needing to be constantly fighting a current (the water flow) and that fatigues the fish?  So then if the flow is maintained but allowed to be diffused or spread out, then would the fish not feel as much water flow and not be stressed out? 
2. Or is there still a problem with too much overall water flow (no matter how evenly diffused it is in a tank) and fish just can't handle it b/c it impacts their gills and gas exchange or something?

I'm particularly, curious about the impact of water flow for smaller fish like platies or white cloud minnows.  Would the 5x tank capacity rule ((e.g. 20g tank means 100gph pump) apply to these smaller fish?  Or less flow is needed?

[Flumpweesel]

I just watch the fish behaviour if they look like they are struggling or unable to stay in one place without a lot of effort then I dial back or baffle the flow.

The size of fish does not really relate the effect of flow as much as fin size in my opinion. Whiteclouds and danios certainly seem to thrive in water with a good flow and and found in such in the wild. Faster water can be better for O2 exchange in fish this can sometimes be compensated with airstones to enrich the water

Larger fish can have more issues in strong currents as it is much harder for them to get shelter from the effects.

 

Sorry if this is a bit of a ramble it's late and I'm sleepy. Hopefully someone more succinct will step in 

[Patrick_G]

That’s a good rule of thumb for beginners, but @Flumpweesel’s advice is better. Keep an eye on the fish and make sure they have some space in the tank to get out of the current. Also keep up with your testing. In many cases you don’t need need the 5x turnover and  your parameters will stay stable with less.