I have a 8’ long x 24” deep x 31” tall tank.
two Gyro 350, one on each end. What is recommend settings? Placement? I use the auto programs but they are blowing my sand bed away.
What sand are you using?
Where are they placed in the tank?
What size tank? (Edit... I see measurements)
I would suggest turning then down a little at a time until sand stops moving.
Or
Replacing sand with larger grain sand
Or
Buying larger grain sand to fill in bare spots.
I have 2 x gyre 250's on my standard 120DT. I have them on the back glass about 1/3 of the way down from the top and angled to push water up a little. They max out at about 70%. Even then I ended up replacing my sugar fine sand with special grade.