Honestly, they look like the grommets I'd document used in holes drilled in steel boxes for the purposes of protecting low-voltage electrical cabling.
I never envisioned that type of grommet used to protect against water leakage in a hydro system
That said, building a hydro system (RDWC) is something I've always thought about (and have base plans for), but that's not for at least a year yet.
Personally, at the base of a system like this, I'd want far larger than 1/2 to 3/4" returns, but I digress.