Hi, You never know if hes gone or not, I have had many things vanish and reappear weeks or months later to my surprise, including a Scott's Velvet wrass I gave $125 for...into my tank..under the sand in 1.1 second...never seen again...2 years later I was in the process of moving, Guess what? My 1.5 inch wrass was now 4 inches long...stunningly colored..and hiding under my sand bed. Goes to show! As for your snails, if you feed your tank often as you have fish too, they should be fine. They will scavenge carrion (fish waste and excess food), they wont eat algae, can't say about diatoms from experiance, Ilynassa Obsoleta will eat diatoms, algae and carrion, even cyano from what I have read and experianced from others in the hobby that I trust, in fact I am ordering a bunch from a private collector friend of mine now, I am even thinking of selling them, but one step at a time. I wish you luck finding your cuke my friend...LOL