A fully gown, ~50cm SL BGK has no place in a 75g, these fish need something far more in line with a 300x100x100cm tank (given they have pretty inflexible bodies to turn corners, something wider and deeper would be better).
That size tank is fine for African Brown Knife Fish, which do well to reach ~20cm SL in the hobby (mine is ~14cm after owning it ~18 months, bought at ~12cm SL).