Not technically a fountain pen, but the Karetake No 13 brush pen has a refillable converter and takes fountain pen inks.
This is only my second ink, but I am amazed by it so far! It is a vastly wetter ink than the Karetake black cartridge, and it bleeds through quite a bit cheaper paper, at least with the bath that a brush pen provides. My Leuchtturm1917 is robust enough for it, though. So far, I’ve been pairing it with iroshizuku murasaki-shikibu (Beautyberry) in a Lamy Safari. It is a lovely contrast in my opinion.
I need to get this sometime! I’ve always meant to; it’s such a lovely color.
It is spectacular. My wife thinks that it is kinda ugly, but she also thinks that I’m good looking. Her taste has been suspect for a while!