What have you filled your fountain pens(other than fountain pen ink) with, and what were the results? Good? Bad? Disastrous? I've tried in the past: Printer ink, diluted with some water, and it worked excellent. Dries quickly and is quite water resistant. Calligraphy ink, started out okay then failed. Clogged up the pen after a while. Ink from re-hydrating the core of a washable marker, works just like FP ink. Made it a bit too light though. FP Ink mixed with methanol, works sort of. Line width increased by 3-4x, some fuzziness and dries absolutely instantly. FP ink mixed with isopropanol, works similar to methanol. Juice, yes fruit juice. Very light, works kinda but to light to be usable. Didn't keep it in the pen long enough for it to become sticky. Blood, just dipped, seems to write okay for a couple words. Didn't put enough in to really know. Plain water, plain methanol, plain household ammonia, absolutely useless as expected PS: Don't worry, I have plenty of fountain pen ink. Variety, you know?