It is normal to have a pH between 7.8 and 8 on a fresh saltwater mix.

In general pH is an indicator of how acid or alkaline the water may be, a far more accurate indicator is the actual carbonate hardness of the water (Alkalinity).The Alkalinity level within the correct range for reef aquaria i.e. between 7-11 dkh will show that the carbonate buffer system is being maintained at a level high enough to resist the addition of acids such as fish wastes, biological processes etc that may otherwise suppress aquarium Ph by reducing or ‘using up’ the carbonate alkalinity.

As the water goes though an RO membrane it is stripped of Oxygen meaning that when you add salt to the water you will find that the remaining high dissolved CO2 levels within the RO water can upset the carbonate alkalinity balance of the mix. Carbonate alkalinity is made of of carbonates and bi-carbonates, If we dissolve gaseous CO2 into the water the carbonate alkalinity becomes biased to the bi-carbonate phase. This is because some of the carbonates have ‘soaked up’, available CO2 which is an acidic gas and become bi-carbonates.

When carbonate alkalinity shifts to the bi-carbonate phase there is a temporary shift in pH downwards even though the total alkalinity is fine and within spec, you just have more bi-carbonate than carbonate. Once the water has been given enough time and agitation the water will come into equilibrium with atmospheric CO2 and levels of carbonates and bi-carbonates will balance again whilst total alkalinity will remain the same over all. If alkalinity was to shift to the carbonate phase by adding something like sodium carbonate to the water the pH would go up, although total alkalinity may only increase by a small amount.

So in short if alkalinity is good you are seeing a temporary shift in pH whilst the carbonate to bi-carbonate levels balance and there is nothing wrong with the mix, the carbonate/bi-carbonates just need time to balance again. As an added note if you mix in an area where atmospheric CO2 is high the pH of the solution may remain low as it has come into equilibrium with high ambient atmospheric CO2 and it will be biased to the bi- carbonate phase.

Mixing the salt in too fast into warm RO water can also shift to the bi-carbonate phase as you may induce a little calcium carbonate precipitation which again can affect carbonate alkalinity balance temporarily. Always mix the salt to cold RO water and when mixed bring up to temperature, you may also find that aerating the RO may offset the pH reading prior to mixing by gassing off CO2 but the water will naturally reach equilibrium eventually.

Once added to the aquarium you will find that the pH will balance with the aquarium water due to the high levels of gas exchange at the water surface providing that there is good surface agitation and the CO2 levels in the room are normal.