A BB tank means Bare Bottom, or glass. I personally don't like the look of that type of tank and feel it is wasting a lot of real estate that could be growing bacteria and pods.
It was Robert Straughn, "The Father of Salt Water Fish Keeping" in the 50s that "discovered" undergravel filters in salt water. He didn't understand the role bacteria play in tanks so used that filter as a mechanical filter and cleaned it when it clogged.
That worked then but we didn't have reef, just fish tanks so it wasn't a big deal to remove the gravel for cleaning. I devised the Reverse Undergravel Filter in the 70s and it is still running in my tank with no problems or issues. The longest I had that filter running was 40 years. I lifted the UG plates mostly to see what was growing under there and it was mostly tiny, red feather duster worms, bristle worms and pods.
I think 40 years is to long and I should probably have at least cleaned it a little after maybe 25 years to allow the maximum flow. Mine is now only running 5 years now because I moved here 5 years ago. I put everything in large vats and re assembled it all here myself when I was 70.
I think it's 53. I'm not sure but I started it in March 1971 so maybe someone can do the math.