what do powerheads do? i know they create a current but like are they good for the tank? i have a 75 gal which is 4ft long aprox. i plan to put a small stingray in there. i got a powerhead from a friend but not sure to use it or not. i put it in the tank and it is on an angle. it didnt come with a mount so its on the top tilted downwards. it right next to my filter. my tank is cycling right now so i have no fish in there yet. are they good for stingrays? should i have it on highest setting? is it ok to be on an angle? or does it have to be straight? it a barebottom tank if that helps.