I don't think it is a sea snake simply because out of the time he has been in the tank he has only come out once. Also Garden eels tend to sit there with their heads out most of the day...he is never out, also like you said he has tiny eyes.
Give me some time to try and catch him again, i will have to dig through my sand for him and try and stop the fish from eating him...or my arm for that matter. I'll drop him in a bucket and try to get a couple more pics of the face, possible gills, skin texture and/or tail again. My camera isn't that good but i will do my best.