help with reverse ugf where pre made tank has water sucking under gravel to the over flow

Hi Everyone,
pls help:i have a 3m reef tank,4xmh lights, chiller 25*c,2m cabinet sump coral medium.i get my sand coral & water straight off the reef(live in Bali)& change about 100ltr/wk fish/coral/ph etc all good.BUT hard 2 keep sand clean!Tank pre fab china.back left cnr of tank is over flow & at bottom of it @ bottom level it sucks water up.if i dont have coral chunks(like in sump)infront of this 2cm suck up part my water level goes up to the rim where the over flow is & wont allow any space- top of glass meaning top of water stays still @ other end of tank.i need the suck up to work & sand up off the glass bottom in the tank maybe?bought plastic kennel grates/grill 4 under sand+mos net plan 5cm coarse new sand off reef,not sure if i should make reverse ugf or let sit still & just put coral bits near suck up?if i reverse my reverse 1x a month i think i cld such up the crud under grill?& once every few months suck up old sand replace with new?but i dont want to have to un build all again!too hard as u know!ive done a complete swap of tanks about 6 months ago from my 2.2m one to this one & only lost 1 fish!have tangs,angels,damsels,golbies,etc

Welcome to our forum. I don't know much about sophisticated filters, but maybe another Aquarium Club member will answer your question. You may want to reconsider taking sand directly from the ocean, because it could contain diseases, toxins or parasites. I'd love to see pictures of your aquarium and of Bali.