The BN's I bought were 3.5-4 inches each {full-grown }and covered 1/2 of the sand in poo in 3-4 days. 75 Gallon! You are in for some poo!
Even fixing my water flow problems just made a insane pile in a corner. I will have them again, but on a dark sand, in a big tank, where the poo piles up where I can suck it out every few days.
Both of mine are small, here's a pic of one of them:
