Additives and Trace Elements
Marine animals, especially invertebrates, from corals to snails and crabs consume many trace elements and minerals in your reef tank just like fish do food. Just like you might take vitamins and minerals to supplement your health and well-being. Various minerals and elements must constantly be replenished, like in feeding the fish as they are being constantly taken up by your charges. For instance, corals are always trying to get calcium out of the water to grow if they are healthy. There are numerous chemicals, elements, minerals, etc., that all the occupants of your tank are competing for. Seawater has in the high 60's or low 70's different elements in it, most in trace amounts, a few in higher amounts, like calcium.

And so for marine invertebrates especially, from iodine to strontium, magnesium to minute amounts of dozens of elements that normally naturally occur in the ocean (saltwater) they are being used and must be replenished. In a closed system, we must add these elements or minerals to the tank to make up for what is constantly being taken up by our animals.

You need to create a schedule and regimen whereby you routinely add a series of items to ensure your tank is complete with all the chemicals, elements, and minerals the animals require. Some people use dosing pumps to put small amounts in the tank at regular intervals.

If you are new to this and can't keep track in your head of which of 15 tanks need which additives, I suggest making a Word document, text Notepad, or even better an Excel file you keep on your computer with the names of the additives on the left going down, and across the top date a new column every time you do it, and go down each item and note amount of dosage. Also you can have some lines for temp, pH, or test results of each thing you test for, and have a really, really good database of your aquaria's vital statistics. Use a different sheet for each tank.

You may not think you need this, but if something ever goes wrong, (or if something goes really right) you'll wish you had this data.     It will help train you with good habits of good aquarium husbandry. Just as you may take vitamins regulary, your tank also needs supplemental nourishment. Remember ... calcium grows bones, fish bones, coral bones, etc.! However do not add milk to your tank, or feed multi-vitamins to your fish.  

As for what to add, overall, most of the products out there are fairly similar. You'll see when you add iodine, usually within a day or two all the crabs and shrimp have molted. This is how they take up new iodine. Likewise you can see similar results from corals with increased polyp activity or color, and vigor.

Calcium, magnesium, strontium, molybdenum and iodine are all very important. I have for most of my tanks and systems over the years used a general all purpose trace elements additive as a basic all-around additive weekly or bi-weekly at the minimum. Then I add small amounts of other things as the above mentioned elements bi-weekly to monthly, pending the bio-load in tank.

To name a couple of additive brands from personal experience, I've always been quite satisfied with anything Sea-Chem I've ever used. Lugol's Solution is another very good product and a good dip for new incoming stuff can be made from it as well. Kent Marine stuff seems fine too as is Warner's. Again, I think most of what is out there is very or fairly similar to a large degree. There are guys that test it all that know lots more than I about each type specifically. I'm not a retail dry goods guy so not studied up on every type.

Get what is available easily for you. Build a regimen up and stick with it. Like temperature or salinity, which you go with isn't as important as the consistency of it. But don't think you can skimp and save by not add anything to the tank for months on end, as some of the animals will be literally starving for strontium, or iodine, or molybdenum. Yeah, I think the stuff can be a bit high in price for a bottle of juice that is likely lots of H20, but we do have to have it to provide the best environment possible for the animals in our care.

They didn't tell you that when they sold you that setup?  

Have a happy habitat!