How corals look at 0.44 ppm PO4 and 5.5dkh

[KonradTO]

Hi all,
I started to test my 15g after almost 1 year. The tank was created as no-hassle "pest tank" where I would keep things that I did not want in the 32g and not stress too much, but now ended up being my main tank as I took apart the 32g.
When I have seen my acro not showing much PE for some time I decided to test and I found:
5.5 dKH (ATI professional)
NO3 25ppm (Salifert)
PO4 0.44 (Hanna checker ulr)
Ca 450 ppm (red sea)
Mg not tested

Numbers are quite stable, as I tested few times and checked the values with a reference solution.

Corals look happy as you can see, maybe not growing much but are definitely alive. How is this possible? All I knew was that most corals would suffer with such numbers. Since they are happy I am not making quick changes and just raising Alk to 6-7 over the next few weeks (I already moved from 5.5 to 6 in one week) by dosing soda ash and AFR. Also I am trying to lower PO4 to 0.1 very slowly. For now I am using some GFO PO4 remover, added some more macros and increased water change. I think the reason behind the high nutrients is the little sand I moved from my 32g to the 15g without rinsing.

 

[Timfish]

Nice tank!

First off, corals can acclimate to conditions that might normally kill stuff not acclimated to the conditions. Additionally, what we can test for is just what's left over and may not be representative of what's being generated and consumed. Alkalinity, for example, can be generated by both endoliths dissolving carbonate substrates and sponges converting DOC to DIC. Research has shown the PO4 levels often reccomended are much lower than corals normally see in the wild. Here's some links if you're interested:

Nikko Reef in Palau has alkalinity similar to yours and is considered relativley pristine

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013GL058489

Coral Reefs in Palau Surprisingly Resistant to Naturally Acidified Waters

Ocean researchers working on the coral reefs of Palau in 2011 and 2012 made two unexpected discoveries that could provide insight into corals’ resistance and resilience to ocean acidification and to aid in the creation of a plan to protect them.

www.whoi.edu

PO4 stuff

PO4 levels on reefs, see table 3,

Environmental Limits to Coral Reef Development: Where Do We Draw the Line?1

Abstract. SYNOPSIS. Understanding how reefs vary over the present ranges of environmental conditions is key to understanding how coral reefs will adapt to

academic.oup.com

Effects of phosphate on growth and skeletal density in the scleractinian coral Acropora muricata: A controlled experimental approach

Phosphate contamination can negatively affect corals, modifying growth rates, skeletal density, reproduction, mortality, and zooxanthellae. We determi…

www.sciencedirect.com

Phosphate deficiency promotes coral bleaching and is reflected by the ultrastructure of symbiotic dinoflagellates

Enrichment of reef environments with dissolved inorganic nutrients is considered a major threat to the survival of corals living in symbiosis with din…

www.sciencedirect.com

ALkalinity and calcium stuff

The three steps of the carbonate biogenic dissolution process by microborers in coral reefs (New Caledonia) - Environmental Science and Pollution Research

Biogenic dissolution of carbonates by microborers is one of the main destructive forces in coral reefs and is predicted to be enhanced by eutrophication and ocean acidification by 2100. The chlorophyte Ostreobium sp., the main agent of this process, has been reported to be one of the most...

link.springer.com

https://core.ac.uk/download/pdf/226900761.pdf

https://pure.rug.nl/ws/portalfiles/portal/14555035/13completethesis.pdf (See ch 4)

 

[Randy Holmes-Farley]

5.5 dKH is probably close to the minimum they can take take alk down to, and they are alive but not expanding their skeletons much.

Many corals can grow fine at 1 ppm phosphate ad 100 ppm nitrate. Might they do better at lower levels? I expect so, but there's not a lot of good studies in reef tank settings.