The reason I bought Noodler's Gruene Cactus Eel was because I wanted a dark green ink that I could use for writing. It had to be dark enough and saturated enough that the words would show nicely on paper. In this regard, the ink performs. The "Eel" line of Noodler's inks refers to the fact that the ink has lubricating properties. Noodler's also has the same ink in a non-Eel version. I thought that it wouldn't hurt to get the lubricating ink since the fountain pens I use aren't very high end and it should help with the flow in some of them.
Edit: Brian Goulet of Goulet Pens points out in the comments that, "A note to add about the 'Eel' properties...it's not just a lubrication for your writing, but it's actually designed for piston-fill pens, to lubricate the piston mechanism inside the pen." Thanks Brian! I wasn't exactly aware of that.
As a parenthesis, I planned to use this ink in a Sailor HighAce Neo fountain pen with F nib.
The Gruene Cactus Eel comes in a standard Noodler's 3oz / 89ml bottle and retails for $12.50 which makes it $0.14 per ml, just like most other 3oz Noodler's inks.
In truth, I can't tell if the Eel lubricates better than a regular ink. It flows well enough in my Sailor HighAce. Unfortunately, the nib is a little too fine for my taste and produces a very thin line.
As you can probably see from the samples, this is a highly saturated, dark and pure shade of green. It reminds me of a coniferous forest.
This ink doesn't show much shading. It is rather flat, although if you squint you will be able to distinguish just a little bit of shading in the sample written with the thick dip pen nib.
The Gruene Cactus Eel is well behaved in regards to feathering, even with a thick point such as the J Herbin glass dip pen. It doesn't bleed at all through the photocopier paper and very little through the journal paper.
To be honest, I am starting to believe that the Staples journal I use for testing inks has a lower quality paper than the photocopier paper I'm also using. You can see some evidence of this in the drying times. On photocopy paper it takes a very long time for the ink to completely dry. On the journal paper it dries very quickly. I'm guessing the latter is a lot more absorbent and this usually means the paper is not top-notch quality.
If you are looking for a water resistant green ink, this is not the one. As you can see, 30 seconds under a running faucet erases almost every trace of the ink, although faint lines remain.
Unfortunately, I can't find any use for this ink as a daily writer. Although the color is ok, I'm not ecstatic over it. I'm not saying the color or shade are ugly but it simply doesn't make my heart beat faster when I use it. I do use it for drawing though.
In retrospect, I regret buying a whole bottle. I should have got a sample first but I will try to make the best out of it by using it for drawing and sketching.
To summarize, Noodler's Gruene Cactus Eel is a dark green, saturated ink, with lubricating properties. It is fairly well behaved in most aspects but I'm not happy with the dry time on less absorbent paper and it is not waterproof. It doesn't make my heart race but it might make yours.