Used to fill every gap and crack I could find in the house and basement cut my heating costs in winter in half roughly.

I bought this stuff several years ago to use on my rock structured waterfall on my pond. When I started the waterfall this spring I was losing a lot of water and suspected that the cause was gaps in the rock stacks so I grabbed this stuff out of the cabinet and tried it. In short, it's great! It expands a lot so think about where you're shoving that nozzle, but it fills in and you can cut off the excess with a serrated knife. It sticks to everything, especially your hand so use some vinyl gloves. I was able to clean up the nozzle, the tube on the applicator and my hands with acetone (my hands not so well so I recommend gloves). You don't have to dry out the surface just turn off the waterfall because it can't be applied in running water. I'm very happy with it.
The exposed spots from the original waterfall building are aged to a nice black and fit in nicely like dark moss (shown in photo). Make sure that you shave back any exposed parts with a serrated knife to get rid of that shiny grey/purple look and try to hide it with rocks.
I read some of the negative reviews and honestly I can't imagine what they're using it for to get those results. It's called Great Stuff Pond & Stone, so it works great for this application.

The good:
This expanding foam is black in color, so it hide / blends well into natural landscaping decor. It if formulated for ponds and terrariums, so it does not contain chemicals which are harmful to plants and animals. It helps hold objects together while providing some degree of water proofing / resistance.

The bad:
This is about 2-3 times the price of yellow expanding foam. Directions state is is "fully set in 8 hours", "cures rigid" and "can be cut, shaped, and sanded". This is not exactly true. This foam expands very slowly for a couple hours. So you will spray it into a shape or between a couple objects and when you return 4 hours later it will have completely pushed the objects into different spots due to expanding pressure and it no longer resembles what you were looking for. Then after it cures when you cut and shape it, it spends then next 24 hours collapsing and deflating. So, you need to wait about 3-4 days before you carve, then take it slow and carve a little here and there over a couple days to get the desired shape. I sprayed a layer of foam covering a 3' x 3' area. After I carved it, it lost about 6" in length and width as it shrank back down. Also, it is not as rigid as you would expect, it will bend and twist to some degree.

Summary:
There are not many options if you are looking for aquatic spray foam that is safe for plants and animals, so this is probably your best bet. But depending on application, give it a long time to cure and take it easy with the carving. Be prepared to spray layers to fill voids created when it re-shrinks.

Attempting to use this product to make a fake rock wall in a small fish tank. The project so far hasn't turned out so well. I intend to try again with some more of this stuff. It's definitely a learning process. My suggestion is to apply just a little bit and wait for it to expand. Though once you begin to use this and stop, the stuff hardens in the end of the applicator to the point of no return.
Also this stuff will stick to your skin and lord help you if it sticks to the hairs on your forearms (or anywhere for that matter).
Also not super water proof. Also used this as a separator in a tortoises wet/dry tank and had to go back through with silicone to patch up some holes that weren't even visible.

This foam works better than others I've tried, especially in its application. It starts a steely black color, but turns a more natural looking gray after it cures and ages a few days. I most recently used it to seal a couple areas where our pondless waterfall was losing water that backed up among the rocks then seeped out over the liner. I have also used other versions of Great Stuff to close gaps in other outside areas of the house. I do recommend that you plan your project so that you use a whole can at one time as it is difficult to keep a used can "fresh" for very long.