Overall, in our opinion, not much.
Still fish and corals in a glass (or acrylic) box filled with water. For the animals to survive and thrive, the goal is “clean”, balanced saltwater and the strategy as we see it remains the same, water changes.
The frequency and amount of change depends on your bio-load, your acceptable loss policy, and budget (time and money).
For us, the sweet spot was 10 to 20%, every week or two.
Why are you testing? What are you testing? How are you testing?
Just start with your fish. The canary in the coal mine. How do they look? How are they behaving? Look for clear eyes and healthy fins.
If we had to do just one thing — water changes.
In our 180L system, we have been following a 5 gal water change per week schedule which translates to roughly 10% of system volume (taking into account – for live rock and + for sump) per month.
Assumption: Water quality is always degrading.
By that, we mean as far as the animals in our closed system are concerned, things are always getting worse! The good stuff like essential minerals and trace elements are being depleted, and the bad stuff, pollutants, heavy metals, are building up.
A number of small water changes are slightly less efficient than a few large changes. Published studies indicate for example that one 30% change is the equivalent of a 26% “continuous” change. With many small water changes you are obviously removing a little of the new water.
The advantages of small water changes include (1) less stress on the animals, (2) usually easier to do, so less stress on the reef keeper, (3) doesn’t change the water levels in the tank, (4) can often be done without shutting-off the return pump(s).
DISCLAIMER: We are not commenting on or making assertions about the marine biology, chemistry, physics, thermodynamics, biochemistry, or biophysics of a saltwater aquarium — a closed system. 🙂