Not the best photo but I'd guess its possibly a wild caught. If your stock in the 120 is going to be just a few inhabitants. I think the endlicheri will be fine. Regardless of whether its wild or captive, generally in endlicheri grown out in captivity from young will max out around 18"-22" (generally wild caught will reach the upper limit of those sizes). Ideally a slightly longer tank would be nice when its 18"+ but I'd say it would do fine. For the most part, bichirs don't really swim around much. As for how long you have before moving out of the 75g, that's hard to say. Growth rate will vary with individual and depending on the care provided.
