I really only do them to clean something, like vacuum the sand, or clean the overflow, or other things. I have done them also, to reduce aluminum levels. As far as using water changes to maintain chemistry, the only solid answer is, it depends on your tank. Nobody can tell you, for your tank that water changes is enough of an option for maintaining minor and trace elements. As JDA said, major elements (the top 3 for sure, calcium, alk, and magnesium) are almost always needed to be dosed on top of water changes.
After that, let’s say your tank consumes 10% potassium, well a 5% addition through weekly water change is obviously not going to cut it. Every week, you are losing 5% that the water change is not keeping up with, and thus you should dose. For some people, water changes is enough to maintain these elements, maybe they have less demanding corals, maybe they do larger volume water changes, maybe they do more frequent or automated water changes, maybe their salt has higher amounts of these elements, maybe they don’t actually know and are just assuming water changes is enough without actually testing. Whatever the case might be, only your tank will tell you what it needs.
It is no where near good enough for someone to tell you, just do water changes for minor and trace elements supplementation and don’t do additives. BTW, not all additives cost a lot, and also if your tank doesn’t need a lot of these elements. For example, I dose 5ml of iodine a week, a 250 ml bottle will last 50 weeks, at $7 per bottle, because it’s late, let’s just call that .60 cents a month. I can’t get anywhere near the above quote of $50 a month of additives, for everything. And I do between 5 or 10 gallon water changes a week, and it’s not enough for my tank for a few elements like potassium, iron, iodine, manganese.
On the flip side for nitrate and phosphate, the amount of water change it takes to make an impact on these numbers is usually very substantial. Let’s say your nitrate is 50 ppm. When you do a 10% water change, you still have 45 ppm nitrate after the water change, very little change. If you did a 50% water change, your nitrate is still 25 ppm. So, they really aren’t effective as a nutrient export solution on their own, so you have to supplement by using algae, protein skimmers, GFO, organic carbon, corals that are nice and big so they are consuming these elements, etc. Water changes never reduced phosphate alone for me, so I still had to run GFO for a while, and now I’ve moved to macroalgae.
Carbon does absorb some things, but not everything. This is the second area where water changes can help, as a tool in the toolbox. Let’s say your tin, or aluminum like for me, become elevated for whatever reason, a water change is the only way to restore elements like these back to proper levels. However, this requires that ICP tests are used, to give you the data that action needs to be taken.
If I narrowed a list, water changes are a good thing for staying connected to your tank, cleaning and working on the things you should, and reducing the elements that no other method can reduce. After that, maybe, only maybe, are water changes able to supplement minor and trace elements adequately. You would have to ascertain what your tank needs. For my tank, water changes alone are nowhere near enough.
