Few experienced aquarists give much thought to making water changes. It has become second nature. The advanced and experienced aquarists take it for granted. For the beginner, the nuances and attention to water changes can mean a happy fish or a dead fish, :evil: and anything in between. So here is one way to perform a water change to make a happy fish: 
It should be noted that the greater (higher percent) the water change, the more control needs to be made. A 15% water change or less doesn't have that great of an impact on the whole system, so some areas (noted below) don't have to have such strict controls.
WHY ARE YOU MAKING A WATER CHANGE?
Sounds like a strange question, but MANY hobbyists can't answer correctly! There are three primary reasons we make water changes:
a. To remove wastes that are building up. For instance, nitrates. However, there are many more unmeasured wastes being removed. The buzz word is 'export';
b. To replenish minor elements to some degree; or
c. To fix a water problem.
So how much of a water change should you make? Another question not usually answered correctly by hobbyists. These are the conditions you're trying to achieve, and the sizes of the water change to meet those reasons:
a. Stabilize the water. Make frequent (weekly) small (10%) water changes. This does not export a lot of wastes, but makes the water very stable in terms of content. An equilibrium is reached where the wastes reach their max levels and are held there -- that waste which is removed is being replaced within that week.
b. Reduce wastes. Make not so frequent (every 3 weeks) a medium sized (25%) water exchange. This routine keeps waste content low, building it up to medium-high levels, before a drop.
c. Remove contaminant(s) or fix water quality problem(s). Make a frequent (every other day or more frequent, depending on the problem and recommendation) huge (over 80%) water change. This is a 'low tide' condition -- your marine life thinks it's low tide! There is something wrong with the water, or the fish isn't eating and you're hoping to 'shock' it into eating, or its a major removal of wastes and 'refreshes the system.'
d. Balance the chemistries. Ain't gonna happen! No salt manufacturer can be counted upon to make a 'balanced' formula AND there is no way any manufacturer can account for how fast your marine life removes ingredients/chemicals from the water. You don't 'fix' water ingredient problems through a water change, other than to remove the chemicals you shouldn't have added. Read this and understand about balancing water: http://www.reeffrontiers.com/forums/f15/what-water-quality-27575/ In this post, down about 1/3 is the section titled: BALANCE you are explained and given the balanced chemistries you should be trying to achieve. These are NOT natural sea water numbers. Our salt mixes do not make up sea water.
NOW. . .
1. Make sure you're using proper source water free of contaminants (including but not limited to: dissolved organics, pesticides, ammonia, nitrites, phosphates, nitrates, silica compounds, smell, and poisons). If there is any doubt as to the quality of the source water, test it. Artificial salt manufacturers who recommend using tap water as the source water are wrong. The variations on tap water around the world make it such a wide range of ingredients in tap water, that tap water isn't consistently reliable enough for source water for the marine aquarium. Get more information about the downside to using tap water here: Chemistry and the Aquarium
2. Mix the source water, preferably using a submersible (inside aquarium) pump (not an aerator). The water should move up and down in the mixing container, not around in a circle. Choose a mixing place away from household chemicals (i.e., don't mix the water in the laundry room, paint shed, garage (where car and pesticides/herbisides are stored), etc.).
3. Add the artificial salt to the water, in the quantity required to get close to the marine system's specific gravity.
4. Mix the salt according to the salt manufacturer's directions (as to how long to mix). (NOTE: The salt manufacturer should know how best to mix their salt into water, however studies have shown that the most stabilized water is achieved after about a week of mixing. This extended time is connected to gas exchange and the chemical reactions going on in the water between the various salts and the gases in the surrounding air.) In no case would I recommend mixing for less than 48 hours. It has been discovered that bad salt mixes will usually 'show themselves out' within the first 48 hours of mixing. If the mixed salt is cloudy or undissolved in 48 hours, there is something wrong. (NOTE: Some unusual/less common artificial salt manufacturers may depend upon mixing to chemically react their chosen ingredients. Such manufacturers can recommend up to and including 2 weeks of mixing!). There should be no residue left after mixing a quality salt using quality source water.
5. After mixing, check the specific gravity of the prepared water. [NOTE: If the salt mix is not properly balanced to provide the proper calcium, alkalinity, and magnesium, now is an opportunity to bring those chemistries into the desired range with the chemistries in the marine system.] The specific gravity of the new water should match as closely as you can with the specific gravity of the water it will replace. Usually, matching to within 0.001 sp. gr. unit is acceptable. Adjust the new water by adding a little more salt, or adding some source water. If a lot of salt had to be added (more than 2% of what you've already added) then go back to 4.
6. When the specific gravity of the new water matches the marine system water specific gravity, measure the pH and temperature of the marine system and the new water. Adjust pH and temperature of the new water to that of the marine system water. (NOTE: This is one of the places of the biggest common error -- the pH adjustment. The pH of the new and old water must be extremely close -- to within 0.05 pH units, if possible). The temperature of the new water should be no lower, and can be up to 1.5F higher than the marine system water. When making very small (10% or less) water changes, these controls can be relaxed. Making large water changes, these controls are very necessary.
7. Remove the water from the marine system. You can take advantage of siphoning in order to clean out detritus/debris around in the display tank, QT sump, and/or refugium.
8. Add the new water to the marine system.
After the above, it is important to replace evaporated water with distilled water (if you have a small aquarium), or RO/DI or deionized water for larger systems. Maintain a constant specific gravity of the marine system, a constant pH in the proper zone, and a constant temperature.
Some interesting information and things to know:
a. A sudden drop in temperature as little as 2F in an hour can cause a marine fish mucous coating to sluff off or improperly function. This causes the fish to become sensitive to infection and diseases it could otherwise fend off. This is the reason why fish who go through a drop in temperature suddenly become ill or infected. A small drop in temperature is significantly far worse than a small rise in temperature.
b. pH is measured not in 'straight numbers' but in a logarithmic function of the hydrogen ion concentration. A small change in pH number is a large change in concentration of the hydrogen ion. What seems like a small numerical change is actually a large chemical change. Don't be fooled. A pH change of 0.10 pH units is significant to a marine fish that has never known the pH of its home waters to change by more than 0.01 units over the period of a year!
c. Salinity sets up the way the fish's internal chemistries function. The fish's internal physiology is based upon the fish's environment and that environment is the water, how much salt is in that water, and how clean that water ultimately is with regards to its home water quality. A fish can handle a rather rapid lowering of salinity (specific gravity) but not a rapid increase in salinity. If salinity drifts too low in the system, raise it slowly (no more than 0.002 sp. gr. units per day). The fish's internal chemistries need time to adjust to a change in salt concentration in the water.
d. Chemicals to make pH adjustments need to be compatible with the marine system. Small adjustments can be made with fresh and pure Baking Soda found in the grocery store. This is sodium bicarbonate. Too much of this and it throws off the alkalinity and the pH control is pretty much lost. Another useful pH adjusting chemical is sodium carbonate. Less is needed to raise the pH. You can make this at home by heating pure fresh Baking Soda, spread out on a pan, in a preheated oven at 350F for 40 minutes and letting it cool. This will change the pH up (raise the pH quicker, and less is needed and thus less likely to throw off the alkalinity). Other suitable chemicals are sodium hydroxide (liquid or pellets). No one can say how much of any of these chemicals to use. This is in part because no one knows the starting pH and the strength of the buffer of the salt water you made up. It is trial and error. Just add a very little bit then check its effect on the pH of your batch. pH should be checked by a meter, NOT by a test kit.
Making the fish think the water is cleaner but hasn't changed in pH, temperature or salinity is the key to a good water change!
Hope this helps.
It should be noted that the greater (higher percent) the water change, the more control needs to be made. A 15% water change or less doesn't have that great of an impact on the whole system, so some areas (noted below) don't have to have such strict controls.
WHY ARE YOU MAKING A WATER CHANGE?
Sounds like a strange question, but MANY hobbyists can't answer correctly! There are three primary reasons we make water changes:
a. To remove wastes that are building up. For instance, nitrates. However, there are many more unmeasured wastes being removed. The buzz word is 'export';
b. To replenish minor elements to some degree; or
c. To fix a water problem.
So how much of a water change should you make? Another question not usually answered correctly by hobbyists. These are the conditions you're trying to achieve, and the sizes of the water change to meet those reasons:
a. Stabilize the water. Make frequent (weekly) small (10%) water changes. This does not export a lot of wastes, but makes the water very stable in terms of content. An equilibrium is reached where the wastes reach their max levels and are held there -- that waste which is removed is being replaced within that week.
b. Reduce wastes. Make not so frequent (every 3 weeks) a medium sized (25%) water exchange. This routine keeps waste content low, building it up to medium-high levels, before a drop.
c. Remove contaminant(s) or fix water quality problem(s). Make a frequent (every other day or more frequent, depending on the problem and recommendation) huge (over 80%) water change. This is a 'low tide' condition -- your marine life thinks it's low tide! There is something wrong with the water, or the fish isn't eating and you're hoping to 'shock' it into eating, or its a major removal of wastes and 'refreshes the system.'
d. Balance the chemistries. Ain't gonna happen! No salt manufacturer can be counted upon to make a 'balanced' formula AND there is no way any manufacturer can account for how fast your marine life removes ingredients/chemicals from the water. You don't 'fix' water ingredient problems through a water change, other than to remove the chemicals you shouldn't have added. Read this and understand about balancing water: http://www.reeffrontiers.com/forums/f15/what-water-quality-27575/ In this post, down about 1/3 is the section titled: BALANCE you are explained and given the balanced chemistries you should be trying to achieve. These are NOT natural sea water numbers. Our salt mixes do not make up sea water.
NOW. . .
1. Make sure you're using proper source water free of contaminants (including but not limited to: dissolved organics, pesticides, ammonia, nitrites, phosphates, nitrates, silica compounds, smell, and poisons). If there is any doubt as to the quality of the source water, test it. Artificial salt manufacturers who recommend using tap water as the source water are wrong. The variations on tap water around the world make it such a wide range of ingredients in tap water, that tap water isn't consistently reliable enough for source water for the marine aquarium. Get more information about the downside to using tap water here: Chemistry and the Aquarium
2. Mix the source water, preferably using a submersible (inside aquarium) pump (not an aerator). The water should move up and down in the mixing container, not around in a circle. Choose a mixing place away from household chemicals (i.e., don't mix the water in the laundry room, paint shed, garage (where car and pesticides/herbisides are stored), etc.).
3. Add the artificial salt to the water, in the quantity required to get close to the marine system's specific gravity.
4. Mix the salt according to the salt manufacturer's directions (as to how long to mix). (NOTE: The salt manufacturer should know how best to mix their salt into water, however studies have shown that the most stabilized water is achieved after about a week of mixing. This extended time is connected to gas exchange and the chemical reactions going on in the water between the various salts and the gases in the surrounding air.) In no case would I recommend mixing for less than 48 hours. It has been discovered that bad salt mixes will usually 'show themselves out' within the first 48 hours of mixing. If the mixed salt is cloudy or undissolved in 48 hours, there is something wrong. (NOTE: Some unusual/less common artificial salt manufacturers may depend upon mixing to chemically react their chosen ingredients. Such manufacturers can recommend up to and including 2 weeks of mixing!). There should be no residue left after mixing a quality salt using quality source water.
5. After mixing, check the specific gravity of the prepared water. [NOTE: If the salt mix is not properly balanced to provide the proper calcium, alkalinity, and magnesium, now is an opportunity to bring those chemistries into the desired range with the chemistries in the marine system.] The specific gravity of the new water should match as closely as you can with the specific gravity of the water it will replace. Usually, matching to within 0.001 sp. gr. unit is acceptable. Adjust the new water by adding a little more salt, or adding some source water. If a lot of salt had to be added (more than 2% of what you've already added) then go back to 4.
6. When the specific gravity of the new water matches the marine system water specific gravity, measure the pH and temperature of the marine system and the new water. Adjust pH and temperature of the new water to that of the marine system water. (NOTE: This is one of the places of the biggest common error -- the pH adjustment. The pH of the new and old water must be extremely close -- to within 0.05 pH units, if possible). The temperature of the new water should be no lower, and can be up to 1.5F higher than the marine system water. When making very small (10% or less) water changes, these controls can be relaxed. Making large water changes, these controls are very necessary.
7. Remove the water from the marine system. You can take advantage of siphoning in order to clean out detritus/debris around in the display tank, QT sump, and/or refugium.
8. Add the new water to the marine system.
After the above, it is important to replace evaporated water with distilled water (if you have a small aquarium), or RO/DI or deionized water for larger systems. Maintain a constant specific gravity of the marine system, a constant pH in the proper zone, and a constant temperature.
Some interesting information and things to know:
a. A sudden drop in temperature as little as 2F in an hour can cause a marine fish mucous coating to sluff off or improperly function. This causes the fish to become sensitive to infection and diseases it could otherwise fend off. This is the reason why fish who go through a drop in temperature suddenly become ill or infected. A small drop in temperature is significantly far worse than a small rise in temperature.
b. pH is measured not in 'straight numbers' but in a logarithmic function of the hydrogen ion concentration. A small change in pH number is a large change in concentration of the hydrogen ion. What seems like a small numerical change is actually a large chemical change. Don't be fooled. A pH change of 0.10 pH units is significant to a marine fish that has never known the pH of its home waters to change by more than 0.01 units over the period of a year!
c. Salinity sets up the way the fish's internal chemistries function. The fish's internal physiology is based upon the fish's environment and that environment is the water, how much salt is in that water, and how clean that water ultimately is with regards to its home water quality. A fish can handle a rather rapid lowering of salinity (specific gravity) but not a rapid increase in salinity. If salinity drifts too low in the system, raise it slowly (no more than 0.002 sp. gr. units per day). The fish's internal chemistries need time to adjust to a change in salt concentration in the water.
d. Chemicals to make pH adjustments need to be compatible with the marine system. Small adjustments can be made with fresh and pure Baking Soda found in the grocery store. This is sodium bicarbonate. Too much of this and it throws off the alkalinity and the pH control is pretty much lost. Another useful pH adjusting chemical is sodium carbonate. Less is needed to raise the pH. You can make this at home by heating pure fresh Baking Soda, spread out on a pan, in a preheated oven at 350F for 40 minutes and letting it cool. This will change the pH up (raise the pH quicker, and less is needed and thus less likely to throw off the alkalinity). Other suitable chemicals are sodium hydroxide (liquid or pellets). No one can say how much of any of these chemicals to use. This is in part because no one knows the starting pH and the strength of the buffer of the salt water you made up. It is trial and error. Just add a very little bit then check its effect on the pH of your batch. pH should be checked by a meter, NOT by a test kit.
Making the fish think the water is cleaner but hasn't changed in pH, temperature or salinity is the key to a good water change!
Hope this helps.
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