My sea cucumber has shrunk half its original size within the last 6 months. He obviously not getting enough to eat via filter feeding. Does anybody know what I could target feed, how much and how often?
The largest cause of death in sea cucumbers is starvation..and that sounds like what is happening to yours.
Typically they eat what they find dead or alive in your sand-bed, but you need to have an established tank (2 + years old) to have a sand-bed that has enough of this stuff in them.
I would venture a guess that your tank is too new to sustain the cuke, judging from when you started posting here and the equipment you were looking to purchase.
Can you try and rehome him or ask if you can return him to whatever store that sold it to you (and shouldn't have..imo..better they ask you how old your tank is before selling him to you)