I have 3 lifeguard bed filters, for one you cant see it working except only little sand floating around, but its said to have huge surface areas. I use a 900 for my 300 gal koi tank, never had problem with amonia and nitrates but when temp hits 50's, amonia will rise, thats when I slow feeds to one time week, 2 300FB in a 54 coner and 40 breeder, they never had a amonia spike and the fish never got sick. I have heard that FB filters not only colonize large amount of bacteria but it also traps parasites with in the sand making difficult for them to escape into the tank, I just set up a 125 and I have this huge ich problem, I am currentlyu looking to buy another 900 for it. FYI, before I got FB for my koi tank, my koi was sick constantly with fin and ick, have not had a problem for 3 years now. Change 50 gal water every 4 weeks, (7) 8-9 inch koi's.
My personal opinion, FB should not be a primary filter, it operate with a 160 gph pump. it should be only for biological only, maintenance free after installed. Its a must have if within budget. Never leaked on me, but some cleaning after 6 months of salt buildups. I have 1 hanging and 2 flowing out side of slump,
when I started my 40 gal, I jump started the FB in my 54 for 2 months then when setting up I just moves it over my tank cycled with fish in 2 weeks or less. I have never saw amonia spike. 2 years now water change 5 gal q 4weeks, one 4 in yellow tang 2 clowns and intank 6x8x12 cpr refug. coral frags and xenias. MH PC 380 watts. LR 25 lbs, Protein skimmer, WD at 400 gal/hr. 400 gal power head for circulation. BTY wetdry home built, havent changed 6 months, seems like its biologically sound. no carbon use.