I've two modified AC500's on my 75 gal
Both of mine have been modified with Bio-Wheels, I've 12" of Bio-wheel in each ones outflow, I also use substitute media in the AC500's such as Chem-Stars for additional bio-filtration.
I have spraybars helping to drive the Bio-wheels which are powered by a medium sized powerhead (50-170gph with a DIY
pre-filter), I've a Coralife Turbotwist 9watt UV
sterilizer in-line on this sytem.
It stays pretty much spotless and the water chemistry is boring.