Aquarists who run [tag-tec]fish only aquarium[/tag-tec] may not use additives at all. They are really only committed to routine water changes. Additives are mainly the domain of the reef aquarist.
Even with a [tag-ice]reef aquarium[/tag-ice] the additives will depend on what livestock are being kept. Fish on the reef are the same as in the fish only system, they donâ€™t need any. It is the corals, shrimps and certain types of algae etc that may need supplementary additives for their well being.
I run a soft coral reef, which is now over five years old. Nitrate and phosphate levels are so low as to be immeasurable. The reef is pretty well stocked, not least by the growth and spread of many of the corals. It has much tiny life, and this also applies to the DSB ([tag-self]deep sand bed[/tag-self]). The corals and other life need food, of course, but certain requirements must be met for their continuing welfare, even though routine water changes are performed.
As this system is relatively small there are no dosing devices fitted.
The first consideration is calcium. This is maintained at 400 parts per million (ppm), give or take. Soft corals do not demand the same level of calcium as the hard variety. The other life in the aquarium requires calcium also – snails (there are a lot of small algae eaters which have reproduced naturally, I assume the originals came in with live rock), minute shrimps which need to reproduce their exoskeletons, calcareous algae, and small tube worms.
Consideration is also given to alkalinity. This is not only necessary to protect the pH level, keeping it at an acceptable value, but at higher levels assists the decorative algae, and perhaps other life forms also, in their development. I maintain the alkalinity in this aquarium at 4.0 meq/l, though it can fall to 3.5. It is not recommended to go to a higher alkalinity. The decorative algae and its growth is very pleasing. It has been said that nuisance algae does not grow so well at higher alkalinity levels. I donâ€™t know how accurate that is, nitrate and phosphate probably play a more significant role.
I used to carefully add iodine, but decided to experiment by not adding it for three months. This was done and I could not see any difference whatsoever in the growth or apparent health of the corals, or other life in the aquarium. I have not resumed dosing. There are various opinions on iodine dosing.
The hard coral enthusiast has to keep an eye on calcium levels, and may well use a calcium reactor because of the higher demand . Magnesium levels will need monitoring. The system may benefit from higher alkalinity levels also. In addition, strontium and the like may be added, but it should be said that there is much argument over the necessity of these. These do not represent the only additives that are available, though the necessity of these others may again be a subject for argument.
So the aquarist should test to ensure that required levels of known demand on parts of the seawater mix are present in sufficient quantity. Though necessary, routine water changes may not meet the demand. If they do not, additives may be the answer.
It seems reasonable to state that if tests are not done for X do not add X. Additives may be good and necessary, but the level in the seawater must be monitored.