Water Changes: How much and how often?
From most studies that have been done on this subject, the conclusion always ends up being that weekly water changes are best. As this gives a very small margin for the marine aquariums water chemistry to become out of balance with natural sea water(natural seawater, NSW). If you were to be lazy with water changes it would inevitably leave enough time for the water chemistry to change for the worse, and to a point where you then have to do a very large water change that will then impact the marine fish and corals with stress/shock which could have easily been avoided. The amount changed out weekly in your natural sea water(natural seawater, NSW) changes should be enough of a dilution to keep your parameters within the acceptable benchmarks whether that be a fish only or reef aquarium.
For example, say you have a reef aquarium and your SPS/LPS corals start to look unhappy when your nitrates go above 5ppm. Then you will need to do enough of a water change each week to make sure that your nitrates never exceed 5ppm. If you are currently doing 10% a week and your nitrates are still creeping up then you will need to increase it until it sits at just below or on the level you are focussed on. This level will vary from marine aquarium to aquarium.
Please also note that if you are doing larger natural sea water(natural seawater, NSW) water changes, those above 15%, then you really need to make sure that the water is going in slowly in order to not cause unnecessary stress to your marine fish and corals. The smaller the water change, the more margin for error you can accept regarding salinity, pH and temperature. However, you should be aiming to match these as close as possible to your marine aquarium before each water change.
If you are currently having issues with your aquarium and you have not tried our Natural Seawater (Sea Water, NSW), then it would be well worth giving it a go and seeing the improvement it can have on your fish tank for yourself!