From this thread
Hi Lasse!
After growing chaeto really well in our sump for the last few months, I noticed our sump a couple weeks ago looking way too clean, and the chaeto was not growing as rapidly as it did before.
I set the Apex to turn off our Reef Octopus Classic 202-s throughout the day, and back on when main lights are turned down. I also removed the filter sock. So when I feed drops of phyto a couple mornings per week, I’m glad to not have red skimmate sitting in the skimmer’s collection cup, anymore.
So, all this just to build up nutrients so my chaeto will look happy again? It just seems so weird that I’m not rejoicing at the ultra-clean tank.
I decided to answer in my build thread instead - the answer
Normally the most common limited substances for Chaeto (and most other primary producers - the one using inorganic substances for living) is inorganic C, P,N and K. But it is not always the case - there is a lot of other elements in one or another inorganic form that can limit the growth. Iron, Iodine, Bromine, Strontium and Boron are other elements that´s can be critical for macroalgae growth. I use the word "elements". In Swedish - there is a certain word for the basic (single atom substances you find in the periodic system) elements (grundämnen) - I can´t find this in English but it is the basic elements I am talking about - not different chemical compounds.
Inorganic Carbon - C - is a little bit confusing in saltwater. In freshwater aquaria – it is one of the most limited compounds for plant growth – it is probably the same in saltwater - but the inorganic C compound with the lowest energy cost for the primary producers - CO2 - can be limited in a saltwater system - especially if the pH > 8.2. However - saltwater is rich in other inorganic C compounds like HCO3 and CO3. The problem with this is that it cost energy because they need to be transformed to pure CO2 before the primary producer can use it in the photosynthesis process. This transformation need a special enzyme - and it cost energy to produce and use it. Energy that´s need to be taken from the growth process. Of course - most primary producers in saltwater can use HCO3 and/or CO3 as source for inorganic C (transformed to CO2 in one or another external/internal process). This does not mean that they can´t take up pure CO2 directly - they can and my strong believe is that they do that if there is sufficient amount of free CO2 in the water (lower pH than 8.1 - 8.2 (I have no evidences for just this pH, but my experiences indicate this). It means - in theory - that lower pH (more CO2 in the water) can give a higher growth rate because the primary producer does not need to take energy to convert HCO3/CO3 to CO2. Some growth experiences in different pH that has been done at The Maritime Museum in Gothenburg with
S.hystrix indicate this too. pH around 8 - 8.1 give a better growth rate compared with pH over and below. Note - the growth rate was calculated from weight - it includes produced limestone - not only soft tissue. This was in one experiment, with one coral - I´m not saying that this is a general conclusion. However - with some calcifying phytoplankton in the sea - it has shown that they have risen their biomass today compared with the biomass decades away then it was a higher pH in the sea. I can´t find the references now but it was published some years ago and surprised the researchers. To the bottom line - I do not believe that you in a normal saltwater aquarium need to worry about free CO2 levels.
The biological input for you P is normally your food and that you add living things to the aquarium. Sometimes – there could be metal bounded PO4 in stones and sand you introduce to your aquarium. Excluding metal bounded PO4 – the input is always as organic P. The transformer of the organic P of all types to inorganic PO4 (orthophosphate) is the bacterial decomposition of organic matter (faecal matter, dead algae, dead corals, dead fish and other organisms)
The biological “output” for P is production of new living matter through photosynthesis. Total P will therefore always rise in a healthy aquarium if you do not export living matter like macroalgae. The trick with low PO4 in a biological driven system is to keep the organic pool as high as possible and harvest from that. This means that – as many people see nowadays – inorganic P can be limited in a way that it limits the growth of the system.
The input for N (as organic) is with one exception through food and adding living things to the system. The conversion to inorganic N will follow the same pathway as for organic N to inorganic P by bacteria but the primary compound produced by bacterial decomposition is NH3/NH4. This is further – sometimes – transformed by a total different bacterial family (compared with decomposition bacteria) – the nitrification bacteria – to NO3
There is another pathway for N into our aquarium – it is the nitrogen fixation of N2 to NH4 and further to organic N by cyanobacteria. This pathway is normally not used by aquarist today but maybe it will be the next “hot” thing.
The output of N from the system is however very complicated. NH4 will in higher pH be transformed to NH3 (a gas) in some extend. This will leave the system through aeration (read gas exchange and effective skimmers). The inorganic N transformed to NH3 will be converted (of many different bacterial communities) to N2 in anaerobic environment – the most known process is denitrification. It’s the most known process but maybe its not the most common. In nature – the anammox process maybe be the most importance process.
Shortly – the “input” nitrogen not lost to the air will be used as inorganic nitrogen in the primary production. Its also indicate – in a biological driven system – that in the long run – it will be shortage of N for the primary production (read photosynthesis)
Especially with a refugium – there has been shown that the ration between P and N in macroalgae can be as high as 1-80 – it means that the demand for N is huge in a well planted refugium. My strongest believe is that in the long run – all aquaria driven by biological methods will show a shortage (limitation) of N. You need to dose!
Potassium has been discussed many times and my believes is - it is not a major concern for aquarium without zeolites and refugium. Zeolites will bind K and macro algae seems to consume a lot. I can see a down going effect in my aquarium if I do not dose.
Iron – maybe the most common limited factor. Can be used – in the short run - if you have an algae problem in the DT – just strip your water from iron. In the future – I will add some in my top off water and stop when my Triton test detect iron.
Iodine – have been discussed a lot. For me – the fact that macroalgae content rather much iodine is enough for me to believe that it is needed and in an aquarium without WC – it will sooner or later hit the zero level. If you need to maintain the natural concentration or not – that’s another question – but if you hit the zero level – you will probably have problem with your macro algae. Input is food – especially food contenting nori- output is growth of biomass and volatile iodocarbons from macro and other algae.
Bromine has not been discussed very much but it’s a halogen and together with iodine it is important for forming inorganic antioxidants in algae - Algae is also know for releasing bromoform (a Trihalomethane) to the atmosphere.
According to the importance of Strontium and Boron to grow macroalgae – there is not so much information to gain from the web – but one of the things I seen in scientific articles indicate that uptake rate was determined by the concentration in the water – therefore I try to have the concentration of these two trace elements at the same concentration as in the sea.
Sincerely Lasse
). I will run it as an experiment to see if it's possible to keep Dendronephtya sp alive and well for a longer period. But we also have a lot of other corals in the same tank, maybe Acropora for example will respond in a good way too, we'll see. 
