PeterQ

The piece I try to connect is definitely the top lid. The top side of the lid fits the tubing, while the bottom does not. The nano sponge filter I bought comes with an easy flow kit. And I just noticed that on the page of the easy flow upgrade kit, it has red text "Not compatible with Nano sponge filters sold BEFORE December, 2023". That means the nano filter is somehow different after Dec 2023, which makes it compatible with the easy flow kit. Since the top lid is the part connecting with the easy flow kit, it's very likely the design has been changed. And maybe the new nano filter simply cannot connect with an air stone by design. Anyway, I'll contact customer service in case my guess is wrong and it is a defective piece.

I found a photo in a previous post. The tubing fits both the air stone and the filter top cap well. My cap definitely looks different.

I bought this nano co-op sponge filter, a Ziss air stone, and some co-op airline tubing. Tried to hook up the air stone to the sponge filter (the bottom of the top cap) using a piece of the tubing. The tubing fits snugly on the air stone, but it's a real struggle to get it onto the sponge filter. Check out the pic. The tubing's diameter is either the same as or a bit smaller than the knob. Has anyone else run into this? Thanks.