G. polyuranodon requires brackish long-term (usually can tolerate fresh for 2+ before slowly exhibiting problems, apparently) or water with a high pH, at least according to a guy I spoke to at an expo some time ago. I've never personally kept them, but I would assume they would be, like most morays, somewhat aggressive and liable to munch on small, slow tankmates. Would possibly be alright in a 150 long-term. M. dayi gets rather large and heavy-bodied, much like M. erythrotaenia; I personally would not be comfortable keeping one in a 150 for its entire life, although a small specimen can very well live in a 150 for a couple of years. Many Macrognathus species are on the smaller end of the spectrum and would do well in a 150. Not too sure how big Mastacembelus liberiensis gets, but I had one for a couple years and it didn't grow larger than 50 or so centimeters. Incidentally, M. liberiensis happens to be the spiny eel in your profile picture, and that very image is only one of three I was able to find of M. liberiensis (or what I believe to be M. liberiensis, anyhow).
