H2O+ Low pH

Answer:

Hi David

The pH result will be dependant on a couple of things.

1.Method of testing, chemical Ph tests can be highly variable and difficult
to read. If you aren't already using one I would highly recommend a quality
electronic pH meter freshly calibrated with the appropriate Ph buffers.

2. CO2 content. Dissolved  CO2 will effect pH and without good gaseous
exchange at the water surface CO2 content will be higher than desired and
will depress pH . Point one of the pumps at the water surface to help
gaseous exchange and if you haven't done so turn on the skimmer if you have
one. RO water can have a high CO2 content as Oxygen is stripped out by the
membrane, this can effect a new mix until CO2/O2 has reached
equilibrium after mixing well with a pump that will agitate the water
surface.

3. The effect of photosynthetic animals. Once the aquarium has been stocked
with live rock and a lighting cycle put in place algae's,corals,bacterias
and other photosynthetic life will produce O2 as part of respiration. You
will then find
pH starts to rise above 8.0 during hours of light if the aquarium has good
gaseous exchange. It will drop when the lights are out as these
creatures/algae's will stop producing O2 and actually go into reverse
respiration and produce a small amount of CO2.  pH will then swing between
8.4- 8.0 day/night, although the shift is usually no more than 0.2 so swings
between 8.2-8.0 are usually the norm and a healthy pH for a reef aquarium.

As your aquarium is new and devoid of live stock your Ph trend hasn't
settled as of yet so will need monitoring once it has been stocked. If you
use uncured live rock you will probably see a dip in pH whilst this cycles.
Tank cycling
can be an acidic process and can pull down Alkalinity and pH as both are
linked.

Just a quick note on your specific gravity reading.  If you are using an ATC
refractometer that has been calibrated at 20c you will need to look for an
S.G reading of around 1.0266 at 20c (the temp the refractometer should be
calibrated at and the temperature the water will equilibrate to once placed
on the Refractometer). By doing this the S.G at 25c should be correct, salt
water becomes more dense at cooler temperatures so will give you a higher
S.G reading at 20c compared to 25c(aquarium temperature). If you aim for an
S.G at 20c of 1.025 the actual S.G at 25c will be lower.

The above is dependent on the equipment used to test for S.G and its
calibration temperature, this is usually printed on the equipment, example
20/20  (calibrated with fresh water at 20c/ sample to be tested at 20c)

What equipment are you using to test S.G?

Low Specific gravity will result in low salt concentrations which will give
lower than expected levels of Calcium,Alkalinity and magnesium as well as
all other elements. Low pH can also be linked in as if the Alkalinity is low
the pH will also be lower than expected.

Hope this has helped.

Tony