Pterygoplichthys gibbiceps is correct. This fish will (so long as your husbandry is appropriate) reach 24". Your 75g is unfortunately too small, and this fish will require something in similar dimensions to a 300g.
They need to constantly have food available - I recommend feeding yam, zucchini, bell pepper, squash, and other similar vegetables. You can also grow algae on rocks for him to eat, or increase your photo period slightly (do give them at least 10hrs of darkness to maintain their circadian rhythm).
I would also suggest providing some wood in the aquarium for him to graze on. While not a main part of their diet, it can help out with digestion. Provide the occasional prawn or slice of tilapia (maybe every week or two) if you don't have predators in the tank. If you already feed meat to predators then do not worry, he will get food from that. You do not want to over feed high protein foods as it can lead to digestive troubles.
Best of luck with the fish. If you can't provide a proper home I would consider looking for a new home for him ASAP. You're unlikely to get money for him as your situation is very common in the hobby. Maybe $10 at best.
