In reading a lot of the threads, it doesn't seem like many use under gravel filters anymore??? 15 years ago, every tank had one, & the newest thing I remember was reverse flow... blowing clean water in, under the gravel... I have always in the past ran under gravel, in combination with hang on the back, or hanging canister filters... when I restarted a few tanks, recently, I removed several of the under gravel filters, & am counting on my floss for beneficial bacteria, & most have live plants now to help