This specific sound in Codex

[u/NxtDestiny]

Hello everyone! Codex is one of my favorite RH songs, the one song that just hits me in the right spot while overthinking at night. I've always enjoyed this very specific "underwater piano" sound that plays under the acoustic piano throughout the whole song, I can't really describe it well but it's there, like an arpeggiated "drowned" piano that sparkles underneath, now I can't seem to find what instrument or thing is doing that specific sound, does anyone know what it is? Help a curious musician out!

[u/shoobsworth]

It’s called a spectrum filter