use a white background not a green one.
the luminance diffrence between white and black (as opposed to green) gives you much more room to work with when pulling mattes.
lighting is going to be important as well.
make sure your backdrop (white foamcore) is well lit with an even, bright, white light.
Try to use a seperate key light on the ink/tank, this way you can play with light position on your ink and wont muck up the lighting for the background.
I think India ink is the way to go, its worked for me but im sure inks with diffrent density/viscosity might be interesting.. food coloring.. oil.. etc..
be ready to get messy
have lots of fresh water ready to re-fill the tank with, also have a way to empty out the inky water after each shoot. (dont pour it in your toilet unless you have a really dirty toilet already and dont care about having a nasty blackish ring in there)
you might want to try some timelapse shots dripping ink on paper towels / watercolor paper as well, you will get some nice diffusion effects. (i used that in the open of my reel, where the black little octopus leaves some inky blotches)
Found it on here a while back credit to vivode