Not sure to be honest. I haven't the foggiest what the chemical makeup is of those and if there are toxic substances in them. I'd imagine that a quick email to the makers and you'd be able to find out what the risk to aquatic life is. Or do what I have done in the past, get the (usually) freephone number off the packets that state "got a question then phone...." and give a buzz and ask. I know that Darts don't strictly come under the aquatic category, but being 'phibs it should give a rough guide to start from.
When we were building up a background for our sals we used aquarium silicone to hold things together. That's defintiely not toxic to 'phibs.
I think with expanding form wouldn't it be the fumes that may cause problems? If so, I'm guessing that if you leave it a few weeks for it to totally dry and the fumes to dissipate you may be alright.
Have seen a couple of how to's where they have used a cork bark tube cut in half lengthways for the stream bed.
Regarding expanding foam, my understanding from my days in the boat building business is that once it has fully expanded and "gone off" all of the harmful chemicals have pretty much evaporated away. The only advice I would add would be to run the heating in the uninhabited set-up for a week or so to ensure that all fumes have gone.