Although a fish tank is by its very definition an artificial enviroment we can do a lot to make it as natural as possible. I dont mean just making it pretty with plants and nice rocks - I am talking about the conditions for biological process.
Many reefkeepers invest in hugle expensive mechanical filters, I've done so myself, and buy all sorts of stuff to put in them. Bio-Balls, filter pads, charcoal, phosphate removers etc. All have a place in fishkeeping - but are they really necessary? Are they even desirable?
I believe that simple, low-tech solutions can be found to almost all of the problems reefkeepers are likely to encounter by following as natural a route as possible. Using 'live' substrates and aquascaping. Fresh seawater for water changes. Encouraging macro-algal growth. Employing a clean-up crew. All these will greatly enhance the effectiveness of the nitrate cycle and improve water quality far better than any mechanical filter can hope to acheive over any period of time.
In addition, it cannot be underestimated how much easier the maintenance of a 'natural' aquarium is. Filters clog up rapidly and if the cleaning regime is neglected the filter itself will act as a nitrate bank, it may even 'go bad' and dump toxic sulpher dioxide into the tank causing a wipeout. The more stuff you have plugged into your tank - the more work is required to keep it functioning and the more disastrous it is when they fail!