When you happen upon your aquarium when it’s running low on water and don’t immediately have top off water prepared, there are a few things you can do to keep the return pump from sucking in air. For starters, you could unplug it. It’s an obvious choice, but not necessarily something you want to do. After all, you want to keep the water flowing to help promote oxygen exchange and to prevent it from becoming stagnant. If unplugging the pump isn’t an option you want to explore, you could turn off other equipment like protein skimmers and media reactors, which would dump their water content into the sump, causing it to fill up a little. If draining that other equipment isn’t an option, you might just have to wait until the RO/DI unit has created enough for topping off.