10-20% should be changed each time. I've seen studies that do the math, and 5% is almost a waste of time when you consider the amount of nutrients you are exporting compared to the accumlation rate. I have a large volume reef system, and routinely do 20-25% at a time, with nothing but very good results. If you are going through the hassle of doing a water change, make up a little more (>5%) and get more benefit each time.
However, given that, I think the key to not stressing the system is to prepare the new seawater correctly. In this way, you minimize any possible shock of the new water addition. This is especially important with larger volume water changes because if your makeup water is significantly different than the tank, then you will stress the organism within.
Prepare the correct amount of new water in buckets, or a clean plastic trash can. Use RO/DI filtered water. Once the buckets are full, you need to place an airstone in each one, and aerate the (no salt added yet!) water for 24 hours. The purpose of this aeration is to raise the PH of the water. Otherwise, the PH is very low, and when you add the salt, the natural buffers that you want going in your tank will instead be used up in the process of correcting the low PH.
After the 24 hours, add the sea salt to the water. My favorite salt used to be Kent, until I tried the new Oceanic blend. First measure the specific gravity of your existing tank water. Add salt until you achieve the same specific gravity in the buckets. (also remember that the water temperature should be close to the tank temp when measuring salinity to avoid any measurement offsets!)
Now I let the aeration continue, and the newly prepared seawater to age for 1-2 more days (longer is ok too).
On the day of the water change, I bring the temperature of the buckets up to the same temp as the tank. You can do this by heating some water in a pot, and adding it in gradually, or placing ice in a pot, and floating it in the buckets. Once the temp is ok, I recheck the salinity to insure it matches the tank. Now, I'm all set to go.
Turn off your pumps, heaters, and powerheads. Siphon the old water out, and dump the new water in. You may want to insure any sensitive corals are moved to the bottom first to avoid exposure to air. However, most sps (acropora etc.) have no problem being exposed for a short time.
I actually turn my pumps on, and dump the buckets into my sump. The new water is pumped back up into the tank. I typically make up about 40 gallons of new water at a time, and do a water change about every 3-4 weeks. If I let it go longer, I can note reduced coloration in the corals, and also reduced growth rates. It really makes a difference!
Hope this helps. Let me know if you have any questions!!!
ps...I copied and pasted below from a link on the fishstore and more website. It reads a little different...but everyone's got a different opinion on this stuff. Also, I know nothing about your tank size, bio loading, or other useful info...but I think in general, you'll be fine with the above:
HELP on WATER CHANGES
Our experience has indicated that one of the least complicated routines that will assist you in maintaining a problem-free ecosystem is performing smaller, more frequent water changes. The keywords here are routine, smaller and frequent. Whatever you decide upon, just do it--same day, same size every week. Make it a routine! We highly recommend that you try to perform 5-10% weekly water changes (5 for lower stocking levels, 10 for higher levels) rather than 10-20% changes every other week (again, 10 for lower stocking levels, 20 for higher levels)... and 10-20% water changes every other week are definitely better than 20-30% water changes monthly! Try to stay away from monthly water changes if you want true success--regardless of opinions you may read (written mainly by lazy people). Remember consistency is important in maintaining water quality. Larger, less frequent changes disrupt the chemical stability of the ecosystem more than smaller, more frequent changes. Plus, you wonÂ’t have to work as hard to maintain system parameters, such as pH, alkalinity, hardness, calcium, iodine, etc, with more frequent water changes using properly conditioned water. To properly prepare your new water, it should aerate for at least 24 hours with whatever water conditioners your system requires, i.e., salt, buffer, electrolyte restorers, etc. These guidelines are the same for freshwater, marine or brackish systems.
If you get in the habit of performing smaller changes on a more frequent basis, over time as the routine settles in, it actually becomes very easy to do, second nature so to speak. Whereas, lengthier intervals between water changes tend to cause even further procrastination because it becomes such an ordeal. Also, say you do get in the habit of performing weekly 5% water changes (on a 120 gallon tank thatÂ’s only 6 gallons of water!) and get so busy one week that you forget. DonÂ’t fret, youÂ’ll be okay if it only happens occasionally--however, donÂ’t try to make it up by performing twice as much the following week. Stick to your 5%. The system will quickly balance out again over a few weeks.
Always keep system balance in the forefront of your mind. You want to maintain stability and consistency and cause as little disruption as possible. We insist that water changes are a vital and essential part of capturing a thriving slice of aquatic wonder in your home. Long-term success will be very elusive for you until you embrace this methodology--so donÂ’t be lazy here! Remember this is the air your fish breathe. Keep it clean!