I can't remember what sq ft a 50lb will cover, but I have about 100lbs in my 6x18 and it covers it about to the height of the bottom trim. Again, it depends on your goal with the subrstrate and your stock. Give us some ideas of what youre looking for
I just need to cover the styro underneath that's all. I have silver arowana, a flagtail, a NTT and a bichir. I will also put 1 or 2 big driftwood for the bichir to avoid slime sucking flagtail hehe..