fan speed control dangers?

hello, this is my first post and i am new to this but trying to learn. So i have heard from a few sources that running your can-fan below 40% can be "dangerous" to the fan or harmful. the question i have with this is what kind of dangers do these people mean? such as do they mean it will affect the lasting life of the fan, fire hazards, or what else. i inherited an 8in fan from someone but i believe my filter may be to small. the recommended maximum of the filter is 200 cfm..so i was wondering if it would be dangerous to run the 8in 750cfm fan at around 25% of its normal power. any help would be greatly appreciated.

depending on the types of fan "dimmer" you use then yes it would matter. 20$ for the hydro store speed controllers and those are safe to run under 50% power.

ok thank you very much for your help. i have one from the store that i got for around that price. much appreciated

Is a hydro store "speed controller" different from any other 110V-rated rheostat?

theres the variac controller types and the resister types.

resister(rheostat) types lower wattage/voltage

variacs(hydrostore ones) turn it on and off at a preset time reataining voltage and wattage. (safer on electric motors)

Get a Speedsteer.
http://www.thelashop.com/hydroponics-inline-fan-blower-speed-controller-3-setting.html#.UDuiMKDTC9s

even at it's lowest setting i'd guess it's running 30%, and yes you don't want to run a fan too slow, it will burn up the motor.