Phosphates should be kept at undetectable levels in a reef tank. Anything above 0.03ppm will fuel algae growth and inhibit coral growth.
You should always run a phosphate removing media such as RowaPhos, preferably in a reactor, to reduce Phosphates. All the food we add to the tank contains phosphates; also tap water is high in phosphates, especially in rural areas, so good quality zero TDS RO water should always be used.
For more in-depth information: Phosphate in the Reef Aquarium
For the Advanced Hobbyist:
Iodine is a complex substance and the theories behind testing and dosing are just as complex. Rather than me try to explain this here in a few sentences, you need to read the following 2 links to fully understand the reasons behind testing and dosing.
For more in-depth information: Iodine in Marine Aquaria: Part I
There is very little known about the importance of Strontium levels in a reef tank. Some hobbyists believe corals stop growing if levels are below 5ppm, however there is no real scientific evidence backing this up yet. Strontium is found in natural sea water at levels of about 8ppm, so we should aim to replicate the natural conditions our livestock has come from. Anywhere between 5 and 15ppm seems to be recommended. Most good salts should already have these levels and no further dosing should be necessary, however if you find that Strontium levels are below 5ppm then additives can be used at the stated dose on the bottle.
For more In-depth information: Strontium and the Reef Aquarium
Testing and dosing Potassium is a relatively new thing in reef keeping. It has become popular amongst expert hobbyists specializing in SPS corals using ULNS (ultra low nutrient systems). Some have identified that maintaining the correct levels of Potassium (380ppm) have increased colors in SPS corals significantly.
As with Strontium, very little is known about the benefits and negatives of dosing Iron to a reef tank. The main effect seen that people talk about is the darkening in macro algae’s like Chaeto and Caulerpa. Also the possible prevention of Caulerpa racemosa going sexual and polluting the tank. Randy Holmes Farley carried out a small scale survey on the effects of dosing iron and although it was a small scale survey, he came up with figures that suggested Caulerpa was 96% less likely to go sexual when dosing iron than those who were not dosing.
A lot of us are now using the ULNS (Ultra Low Nutrient Systems) with very low nitrates and phosphates, this in turn is causing our refugiums which were once filled with macro algae’s to start receding as less and less nutrients become available. Perhaps if those using ULNS were to start dosing iron we would see a turnaround in the growth and rely less on bacteria to consume the nutrients and go back to letting the macro algae’s take a share of the work?
Another plus side of this would be we may see less micro algae’s appear such as diatoms as the macro algae competes for the nutrients.
Too little is known as to whether over dosing iron to NSW levels will be harmful to other inverts, from the articles I've seen so far, there appears to be no side effects to massive over dosing.
For more in-depth information: Iron in a Reef Tank
Many of us are under the impression that if we have diatoms in our tanks, then we have too high a level of silica. We see countless posts on here every week asking how to get rid of diatoms, but how many times have you seen someone suggest that they test for silicates?
Silica easily enters the aquarium through poor quality water, either untreated tap water or RO water with a higher than ideal TDS due to poor maintenance in changing the pre-filters, membrane and di-resin.
Silica is found in NSW at levels of around 0.06 to 2.7 ppm, we try to replicate all our other parameters to NSW levels, so why not Silica as well?
Silica is not only taken in by diatoms, but Molluscs and sponges also benefit from it. Sponges utilize the silica to form internal structures, called spicules, which help them retain their shape. Molluscs such as limpets, utilize silica, for the growth of their teeth (radula), and a possible theory as to why such molluscs do not live long in captivity is the absence of silica - although still only a theory.
Reefkeeping is moving at a fast pace and I believe it wont be long until we actually see experienced reefkeepers dosing silica into their tanks (you read it here first folks!)
For more in-depth information: Silica in Reef Aquatiums
I hope you find this information helpful and informative,
**All linked articles written by Randy Holmes-Farley