I found an aquarium pump rated 30-60l that i have no use for could i use that instead of an air stone for circulation and aeration in a 20 liter dwc tank?
Use the pump to run the air stone. You need both.
Confused about the kind of pump you have. Is it a water pump or an air pump?
If it’s an air pump, you’ll wanna get air stones to aerate the water. The air that the pumps blow creates bubbles that are too large for your plants to really make use of /to oxygenate the water by itself.
If it’s a water pump, which is what I’m inferring from the question, then you’ll want to get an air pump and air stones for a DWC. Although the water pump will help you, because it can turn the DWC into RDWC (recirculating) - and increase water movement, give you a second res to add nutes to and check water temps/ph/PPMs. I started in 5gal DWCs and DIY’d it into a 3 bucket and 1 tote RDWC system. It’s not the best design but it works.
420 nailed it. Pump runs the stone sir.