Starving cucumber

I have had a cucumber in my tank for 3 years and he was growing really well but all of a sudden he bad started to shrink. This tells me he may be starving. I see him collect sand and poop it out do I know he its processing my sand but maybe just not getting enough out of it. What can I supplement feed him so he doesn't die on me?

Here is a pic if anyone needs it to ID him since I don't know his correct species.

 

I don't think there's much you can use to directly supplement it, but the animals it feeds on can be supplemented by feeding phytoplankton. You may be able to boost their population and save the cucumber.