Yes, some fish would eat the adults. In general, many fish will eat any fish that can fit in its mouth. I still think Endler's would be the safest since they are small and don't usually eat their fry.
Endlers are commonly sold as feeder guppies. They are snake safe. Most Endler's you will find are hybrids. You can usually find a few cheap ones in the feeder guppy tanks at petstores. They do not have thiaminase.The Endler's are beautiful, but I'm a little hesitant to buy them, because I'm not sure about the cross-breeding for the sake of the snake.... because feeder guppies are one of the few fish that have the less probability of poisoning him.
Some snakes like garters and ribbon snakes like to eat fish, but feeder goldies, fathead minnows, and others have thiaminase with prevents the snake from absorbing vitamin b1 (thiamine) and so they can die from eating those fish. Smelt have it too as do many catfish.
I have seen them in PetCo and other chains as well as at my lfs. Its just little plastic grass squares with a weighted base. The fry can sink in between the grass blades and not get eaten.Wow!!! Okay, thank you!
Like absolutangel04 said about "thiaminase". Thiaminase is found in some fish, and is toxic to snakes, which is why they can only eat certain breeds of feeders and fish. I'll look into the Endlers, and see if any of my local stores sell them!
They have such thing as sinking plastic grass? How does that work?
Would something like this: http://www.petsrising.com/acatalog/PlasticAquariumPlants17cmSmallImage.jpg work?
Yep. Thats what I meant.Ahhh, like this?
Yep.
I don't know if thiaminase affects any other animals besides snakes though. I should look into that... I just know about snakes since fish are a common food for finicky snakes, especially wild-caught snakes, garters, ribbon snakes, and various water snakes. 
