The challenge with additives is deciding which ones to use, which ones NOT to use, and how much of them to use.

As a general rule, start out by using the exact dosage recommended by the manufacturer or LESS. Never use more than recommended if you are not experienced. Then wait a period of time and see if the corals respond. You can retest the water and see how quickly the additive is being used up by the tank. With experience you will figure out how much to add. Keep a log book of how much was added, the level when the water was retested, and how the tank responded.

Additives are critical for maintaining corals. New salt water that is properly mixed will have at least some of all of the additives you need in your tank. But these will be used up over time, at varying rates. Depending upon the type of tank that you have, the additives may be different. If your tank is primarily a soft coral tank, then your focus will be less on maintaining alkalinity and calcium and more on maintaining iodine.

If your tank is primarily a hard coral tank aka SPS or Small Polyped Stony corals, then your focus will be primarily on maintaining higher alkalinity and calcium. Soft corals can also benefit from a higher alkalinity and calcium. But its not as critical as with hard corals.

The Best Additive is….

The best additive of all is to make regular water changes with aged Saltwater. New salt water should contain all of necessary elements and compounds present in natural saltwater and will replenish those items that have been used by the corals by the biological processes in the tank. Regular water changes are a great insurance policy to make sure that all elements and compounds are replenished on a regular basis.

Typically the common additives in a reef aquarium are things such as iodine and calcium and alkalinity. At SCI, those of the main items that we add on a regular basis. There are many other additives that you can add such as strontium and molybdenum and a number of trace elements, but actually the number one additive that we recommend for beginners, the simplest way to keep up with additives , is to just do a water change.

When you use freshly mixed salt water, (and I mean aged salt water, which has been mixed up and allowed to set or to aerate) what you're doing is you are adding back all of the different compounds and elements that are present in the original salt water. Some of these elements and compounds may have been used up by organisms through various biological processes and you are replenishing those. If you don't replenish those elements and compounds or even if you miss just one, you can create a situation that limits the growth of the corals. . That's what is called a limiting factor.

Limiting Factors

A limiting factor is the compound or element that limits the growth of the coral even if all other elements and compounds are present in sufficient quantities. For example in the oceans, iron is typically the limiting element in salt water . You might have an excess of all of the other elements and compounds except for iron and without the presence of sufficient iron, the plankton in the ocean will not continue to grow. Even with sufficient amount of all the other elements of it requires for growth, if one critical compound is missing, growth will be slowed or stopped. But don't add iron in your closed system. Typically its not the limiting nutrient in tanks.

You may see a decline in the coral even when a limiting factor is absent. So that is why I think the best method is to do regular water changes every single week. Of course, the percentage of water changes will vary depending upon how many corals you have in your system and how many fish you have in your system. But as a general rule, if you can do a five or 10% water change every week, I believe that would be far superior to doing a larger water change every month. It's a small price to pay for, what is basically an insurance policy.

Soft Coral Additives

With regards to soft corals only, it's critical that you keep up the iodine levels in your tank. Some soft corals, for example Xenia, are prone to melting down. If iodine levels drop too much within your tank, most soft corals will not grow as well and some may die.

Natural sea water (NSW) level of iodine is 0.06 ppm. This is the dosage that is generally recommended to keep in your aquarium

We have suggested from some sources, that you can safely maintain a level in your aquarium several times higher than NSW , although we don't recommend going over it if you are a beginner. Twice the dosage is often considered safe 0.12 by some people, but because it can adversely affect your fish, we dont recommend it . When beginning, stick with the recommended dosage and try to maintain a fairly constant level by adding and testing for it at least weekly.

Stony (SPS) Coral Additives

With regards to SPS corals, iodine doesn't seem to play a significant role. However maintaining alkalinity and calcium levels is absolutely critical to the success of an SBS or stony coral tank. Calcium and alkalinity should always be added together, because these two complement each other. An excess of one can compete with the level of the other. In general you want to maintain calcium for stony coral tanks over 350 ppm and alkalinity level above about 9.8 dKH or 3.5 meq. . (2.8 * 3.5 = 9.8). As you can see, you can convert milliequivalents in to German degrees of hardness, by multiplying milliequivalents times the conversion factor of 2.8 to get German degrees of hardness.

There are many other additives that you can choose to add, such Strontium, Molybdenum, but I would leave that to advanced aquarists. I dont believe adding them separately is necessary if you are doing regular water changes and are using a calcium reactor, which we recommend highly for SPS corals.