But to answer your question, its just a technical issue. I dont think you can lower the height of the water in your built-in sump. But instead, you can increase the height of the water of your main tank and let the water flow in. To do this, buy a bigger piece of acrylic and replace the one that is in your tank right now that separates your tank and sump. On the new piece of acrylic, on one corner, cut a hole. This hole has to match the level of water you have in your display tank. On the other corner of the acrylic, cut a hole thats higher than the water in your display tank. Silicone this acrylic into your tank. Now fill the display tank to the originally planned height. At this point, your sump should still be empty, now fill the sump to the desired level. Now your water in your tank should be higher than the water in the sump. Throw a return pump into the sump. Whatever the amount pump gives out, the same amount of water should flow into sump. If after trying all these and still dont work, you can find a plumber instead. Lol Sent from Singapore Reef Club mobile app