Many modern marine aquariums use ‘natural’ methods as much as possible. This has been achieved by the advances in knowledge that have been made over the last few years.
Some aquarists argue that it is impossible to keep an aquarium without using natural methods and I can see where they are coming from. The point they have in mind is bio-filtration, all those hard working bacteria getting rid of the toxins in the seawater. Without them we’d be in a sorry state. It’s true as well that this filtration is natural. However, if the bacteria colonies are housed in a canister filter then it isn’t completely natural, the home for the bacteria is being provided by our technology. Nothing wrong with that as it stands.
A fully natural method for bio-filtration, staying with the example, is live rock. This rock mainly comes from the wild and has bacteria present. It is a really good filtration media and, unlike the canister filter, within reason the bacteria can deal with nitrate as well. Can’t be bad!
There are new commodities on the market that are designed to replace live rock. This is a rock that appears very similar to the fully natural variety but needs the bacteria colonies kick starting in the same way a canister filter does. One running the new rocks are suitable homes for all bacteria including those that deal with nitrate. This isn’t removing ‘natural’ filtration; the difference is purely the type of rock. There isn’t any electric motor required. It also reduces the demand from the wild, unless the live rock is aquacultured in the wild.
Anyway, getting back on track, there are a few ways that an aquarist could use natural methods. Think of filtration by algae, deep sand beds, plenums, mangroves etc.
The link below is to a website article where natural methods are definitely in mind.