suddenly slow growth in established tank

[jpirnat]

I have a reef tank system that is about 6-7 years old. For the last few years ive had very consistent testing and dosing. I use BRS 2 part but was using kalkwasser up until a year ago. I use dosing pumps for 2part and have noticed needing less and less to maintain levels, and have noticed a slowdown in coral growth. There is some algae but have noticed coraline almost non existent as well.

Basically i have been turning down the time the dosing pumps are on from about 6min x 10 a day (60ml) on 1ml/min BRS pumps to only 2min x 10 a day and levels are staying consistent. This has been fairly linear. I did have a large(er) acro colony die off but there are plenty of LPS in tank that took up (and still do) the lions share of the coral.

I had great growth on everything up until about 9months ago... Shortly after switching to two part from kalkwasser is only change ive made really, anyone else have this issue? Only other change was switching to a digital salt meter which revealed my refractomitor was off. I took PPT on NaCL from 31PPm back up to 35 over the course of a month or so.

Ive heard of some success with replacing crushed coral substrate, as mine is quite old, but maybe it is time for a calcium reactor?

Any ideas appreciated

90G
Alk 10.0
CA 430
MG 1500
AquaIllumination Lighting
NO3 1PPM
P04 near zero

 

[ggfrancis]

I might be wrong. Your tank is too clean. Your nutrients are too low.


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[Timfish]

"PO4 near zero" is a red flag for me. PO4 really needs to be kept above 0.03 mg/l in aquariums. Here's links including links to research done by Southampton Univeristy in England with corals maintained in an aquarium for years.

https://www.cambridge.org/core/jour...reef-corals/AFB1CF4CB68823BD13AD254623FD3C7C#
An Experimental Mesocosm for Longterm Studies of Reef Corals

Phosphate Deficiency:
Nutrient enrichment can increase the susceptibility of reef corals to bleaching (N imbalance):
http://www.indiaenvironmentportal.org.in/files/file/Nutrient enrichment.pdf

Ultrastructural Biomarkers in Symbiotic Algae Reflect the Availability of Dissolved Inorganic Nutrients and Particulate Food to the Reef Coral Holobiont:
https://www.frontiersin.org/articles/10.3389/fmars.2015.00103/full

Phosphate deficiency promotes coral bleaching and is reflected by the ultrastructure of symbiotic dinoflagellates
https://www.sciencedirect.com/science/article/pii/S0025326X17301601?via=ihub

Effects of phosphate on growth and skeletal density in the scleractinian coral Acropora muricata: A controlled experimental approach
https://www.sciencedirect.com/science/article/pii/S0022098111004588

High phosphate uptake requirements of the scleractinian coral Stylophora pistillata
http://jeb.biologists.org/content/214/16/2749.full