Do you have a BV (ball valve) on the out-put side of your pump? If you ad one there and throttle back a little then play with your return to the sump this might help. It's kind of hard to throttle one (out going to the sump) without the other (return from the sump)
On the kalk, I have that tide into my ATO for me no problems YMMV
nice looking tank to make that in acrylic you'd have to go to 1" material