A vital maintenance procedure in nano reef keeping is doing partial water changes on a regular basis. Roughly 10-15% should be changed each week. One must be very careful during this process because any error could potentially harm the reef. But don't let that scare you, it's a rather simple process. There are different water source options, and three main things to check your water for before you add it to the tank.
The source of water for your nano reef and its water changes can sometimes be a mind boggling issue. The most common options are tap water, reverse osmosis, or distilled. In most areas, tap water will not be good enough for nano reef keeping. There are simply too many hazardous chemicals and pollutants to effectively keep a reef. The most common side effect of using tap water is large algae blooms, caused from nutrients already in it. Reverse osmosis water is the most economical choice for nearly 100% pure water. Reverse Osmosis units can be purchased from anywhere between $75-$250 for a wide range of filter stages and output ratings. If you cannot invest the money in a reverse osmosis unit, most grocery stores have dispensers where you fill your own jugs. Usually $.45 - $.50 USD will get you a one gallon jug already filled, and you can reuse the jugs after the initial purchase. Check for information on the grocery store's system for a number to call, so you can make sure it is a true RO system that is properly maintained. If reverse osmosis water isn't available, the next option is distilled water. The purity of this water is pretty close to RO water, often a little better, but it does cost more. The average price is $1.00 USD for a one gallon jug of water.
After you have your water, the next thing to do is mix your salt in. Since typically more than one gallon of water is needed at a time, a good container should be used to mix the water in. A simple solution is to use a brand new plastic 5 gallon bucket, which can be purchased at a hardware store. In addition to the bucket, the only other equipment needed is a small powerhead & heater. Use the powerhead to keep the salt mixing and the water circulating. Set the heater to 80F degrees and let the water sit overnight.
When your specific gravity (salt level) is at 1.023 at a temperature of 80F the water should be ready to add to your tank. One thing to remember when testing your specific gravity with a swing needle hydrometer (the most commonly used) is that they're only calibrated to give a good reading when the water temperature is around 80F. If it is colder, your specific gravity will seem low. Some more accurate choices for test specific gravity are refractometers or floating hydrometers. When those levels are good, the only other thing you should test for is the pH. It should be at 8.3 before you use the water. If it is too low you may need to add a pH buffer.
If all levels in the water are correct, you can go ahead and make the partial water change. Turn off any pumps in the tank that will be above the water level after enough water is removed, and also all sump pumps if you have one. Since your newly mixed water will have the same specific gravity and temperature as the water already in the tank, it shouldn't cause much of a shock for the fish or corals. You will want to do the whole water change process as fast as possible, but take time to slowly add the water so the sand bed isn't disturbed. Once enough water is added to replace the amount you removed, you can turn all equipment back on.
The basic things to remember are: Make sure the newly mixed water temperature & specific gravity match those of the tank, be sure your water is at a proper temperature before you test the specific gravity, and let the salt mix in for at least 12-24 hours. Stress on the nano reef inhabitants can be avoided by having proper levels, starting with quality pure water, and working quickly.
On a side note; improper readings on a swing needle hydrometer can also be caused by salt deposits in the test sample container. Even if you always rinse out the hydrometer with freshwater after use, you will still need to periodically clean it. An easy way to clean it is to fill the hydrometer with vinegar and let it sit for a few hours. Make sure you thoroughly rinse it out after cleaning.