+1 to Erica
Personally I haven't tried it and everyone on this forum recommends it. Like yourself, I used fountain pen inks. Somewhere on this forum, it is said to mix water with the fountain pen ink.
Use a different container to add water to the fountain pen inks (ratio matters.) Fountain pen inks have sulfactant to lower the surface tension, hence, making it flow nicely for fountain pen use.
Adding water dilutes both ink and sulfactant, hence making the ink "less" liquidy.
In the past, I have added sugar, honey, and glycerine. All worked well and when dried, a light shine can be seen at an angle, and a bit sticky. Now I just add a thickener to my ink, gum of arabic liquid.
What appears as "liquidy" might also have to do with the way your nib is prepared.