Sprinkle the absorbent material generously over the spill, ensuring complete coverage. Allow the absorbent material to sit for several minutes to absorb the oil fully.But if it's a lot, place the fish in a separate container filled with some of it's water from the aquarium, make sure to drain the aquarium's water and oil together with the absorbent materials then refill with tap water then depending on how many gallons, put anti-chlorine for the fish to survive