black spotted eels stay under 12in. there's also the black spotted eel which maxes out at 20 inches, it would be fine in a 75 gallon tank as long as you dont put any other significant bio load in there as well. the problem with the black spotted eels is that theyre commonly mistaken for the much larger tiretrack eel and vice versa, So beware of buying from uneducated vendors and petshops.
A senegal bichir is also pretty good. theyre quite popular for their prehistoric nature and theyre easier to keep than spiney eels. Also at under 12 inches, these can be mixed with other non-aggressive fish that wont snatch up all of the food before the eel does.