Oxygen cones represent another way how to dissolve oxygen in the water. If you're currently pumping water back to your tank, place an Oxygen cone between the tank and the pump to saturate the water with oxygen. You can reach oxygen saturation of 25 mg/l with just 1 barG water pressure; higher saturation can be reached with higher water pressure. Then you can mix the oxygen saturated water with the culture water to keep the fish at whatever dissolved oxygen level you choose.
The Oxygen Cones are made out of non corrosive fiberglass.