Your quality of work is their quality of life
I would suggest putting her food on a target at the bottom. Let her spy hop, then when she goes down to feed she will be eating the food off the target. Hopefully she will eventually understand that the source of food is that target. If your tank is super deep or you have no way to get a target to the bottom and then back up, you could try putting a food pole (target) at the surface and you would probably, literally, just have to hold it there for hours until she understands that that is where the food comes from. If those ideas wont work for you this pdf gave me a lot of insight when I was training my elasmo. Good Luck!