adgshop.com <--- buy it there. in your 55 you would need 4 bags, idk about the 50 cube. probably like 5?
pros- packed full of nutrients, plants just love this stuff. you might not even have to dose the water column if you have low light. it buffers your water to soft and acidic, which plants thrive in. its just the best substrate for a healthy planted tank.
cons- theres an ammonia cycle that you have to go thru once you first get it, but its def worth it. and its kind of fragile, so dont beat the crap out of it.