Higher ph means less H+ in the water.
Alkalinity is not a specific thing . It is many things which have the capacity to neutralize acids by adsorbing H protons. In sea water alkalinity includes: carbonate, bicarbonate,borate,oxide,silicone,magnesium hydroxide , hydrogen phoshate and orthophosphate( aka PO4). In seawater at ph 8.0 96.5 % of the total akalinity is carbonate and bicacarbonate.
The relationship between alkalinity and ph is somewhat arcane but uderstandable even though a headache may result. More on that in the morning.
First let's look at how corals calcify. We as keepers of reef tanks use total alkalinity as a surrogate measure to determine if we have adequate carbonate/ bicarbonate for corals and other calcifying organisms to precipitate calcium carbonate.
If a large portion of total alkalinity is borate or something else other than carbonate/bicarbonate our tests will be misleading in terms of coral health.Further higher ph levels will be reached with less carbonate alkalinity(less than the 96.5%)making it difficult to sustain adequate levels of carbonate alkalinity since precipitation will occur at high ph with relatively low carbonate alkalinity due to the disproportionately large amount of other alkalinity.
Carbonate and bicarbonate change back and forth depending on the amount of H+ in the water(ph). Together they are often called carbonate alkalinity and are what we are concerned with since corals and other calcareous organisms need them. Corals take up bicarbonate; not carbonate. They squeeze out an H proton as they form calcium carbonate (CaCO3) from the bicarbonate and the calcium which they also take up.
So, on the one hand higher ph(less H+ in the water) makes it easier for them to squeeze out a positively charged proton when there are less positively charged H+ protons in the water. thus it would seem higher ph might make precipitation and skeletal growth easier. However ,recent discussions suggest higher proportions of HCO3 vs CO3 which occur at lower ph ranges may be more beneficial overall. A shortage of HCO3 vis a vis CO3 might actually slow growth even though more CO3 results in higher ph and alkalinity( CO3, carbonate can neutaralize 2 H protons before becoming carbonic acid H2CO3 while bicarbonate, HCO3 has room for just 1).
Many systems running low or ultra low nutrients shoot for 7 or 8 dkh to avoid burnt tips where calcium carbonate skeletal growth seems to outpace tissue growth. These lower alk values often mean less buffering capacity and lower ph as well. Reef surface waters run around 8.2ph. The range of 7.8 to 8.5 should be sufficient for growth. Below 7.8 and you risk coral skeleton dissolving. Higher than 8.5 and abiotic(non biological) precipitation of calcium carbonate is likely ,including "snowstorms" which not only deposit calcium carbonate crystals every where but also drop calcium and akalinity precipitously These alkalinity bounces can be deadly to sps in particular ,in my experience.
Alkalinity is not a specific thing . It is many things which have the capacity to neutralize acids by adsorbing H protons. In sea water alkalinity includes: carbonate, bicarbonate,borate,oxide,silicone,magnesium hydroxide , hydrogen phoshate and orthophosphate( aka PO4). In seawater at ph 8.0 96.5 % of the total akalinity is carbonate and bicacarbonate.
The relationship between alkalinity and ph is somewhat arcane but uderstandable even though a headache may result. More on that in the morning.
First let's look at how corals calcify. We as keepers of reef tanks use total alkalinity as a surrogate measure to determine if we have adequate carbonate/ bicarbonate for corals and other calcifying organisms to precipitate calcium carbonate.
If a large portion of total alkalinity is borate or something else other than carbonate/bicarbonate our tests will be misleading in terms of coral health.Further higher ph levels will be reached with less carbonate alkalinity(less than the 96.5%)making it difficult to sustain adequate levels of carbonate alkalinity since precipitation will occur at high ph with relatively low carbonate alkalinity due to the disproportionately large amount of other alkalinity.
Carbonate and bicarbonate change back and forth depending on the amount of H+ in the water(ph). Together they are often called carbonate alkalinity and are what we are concerned with since corals and other calcareous organisms need them. Corals take up bicarbonate; not carbonate. They squeeze out an H proton as they form calcium carbonate (CaCO3) from the bicarbonate and the calcium which they also take up.
So, on the one hand higher ph(less H+ in the water) makes it easier for them to squeeze out a positively charged proton when there are less positively charged H+ protons in the water. thus it would seem higher ph might make precipitation and skeletal growth easier. However ,recent discussions suggest higher proportions of HCO3 vs CO3 which occur at lower ph ranges may be more beneficial overall. A shortage of HCO3 vis a vis CO3 might actually slow growth even though more CO3 results in higher ph and alkalinity( CO3, carbonate can neutaralize 2 H protons before becoming carbonic acid H2CO3 while bicarbonate, HCO3 has room for just 1).
Many systems running low or ultra low nutrients shoot for 7 or 8 dkh to avoid burnt tips where calcium carbonate skeletal growth seems to outpace tissue growth. These lower alk values often mean less buffering capacity and lower ph as well. Reef surface waters run around 8.2ph. The range of 7.8 to 8.5 should be sufficient for growth. Below 7.8 and you risk coral skeleton dissolving. Higher than 8.5 and abiotic(non biological) precipitation of calcium carbonate is likely ,including "snowstorms" which not only deposit calcium carbonate crystals every where but also drop calcium and akalinity precipitously These alkalinity bounces can be deadly to sps in particular ,in my experience.
My summer ph runs high topping out 8.5-8.6, so I switch to sodium bicarbonate added to my top off. During winter I have a hard time getting above 8.2. 
