pH is one of the parameters that is important in a [tag-tec]marine aquarium[/tag-tec], and is therefore checked regularly. It could be that the pH is lower than the aquarist desires. There are a few things that can be checked and tried. This may raise the level sufficiently.
The first move is to consider at what time of day the water test was undertaken. If this was shortly after the lighting turned on, the low reading may be because during the night there is a tendency in some systems for the pH to fall. Therefore carry out a further test say an hour before the lights turn off. The pH may well measure higher. If desired, algae (Caulerpa) can be planted below the display aquarium and lit on an opposite cycle to the display aquarium. This will help counter the pH drop.
The seawater circulation should be checked. Gas exchange takes place at air/water interfaces, so if there is insufficient movement this gas exchange will not be efficient. With adequate design, the turbulent circulation within the display aquarium will move seawater constantly to the surface. In addition, any sump in use has a surface and this will help. Any seawater going over a weir will also assist in gas exchange. Well oxygenated seawater is the target.
If the aquarium is very heavily stocked – this applies more to a fish only system – then the fish load may be a factor. Obviously fish consume oxygen, so the previously mentioned gas exchange should be adequate. Also, the life functions of the livestock are constantly trying to push the seawater towards a lower pH. This tendency can be countered by having sufficient alkalinity, or buffer, which resists this pressure to change. To check these two points, watch the fish breathe. Is the gill beat relaxed and normal (different fish have different gill beats)? Or is it laboured? This is a rather poor check but can give an indication. There are oxygen test kits available which may help. Alkalinity test kits are readily available.
If the above does not help, then carry out a pH check well after the lights have come on. Then take a gallon or two out of the aquarium and put it into a seawater-safe bucket. Run an airstone in the seawater for say two hours. Maintain the temperature with a heater. Measure the pH, has it risen? If so, aquarium gas exchange is inadequate. If not, take the two gallons of seawater outside into the open air and leave with the heater and airstone running for a further period. Test the pH, has it risen? If it now has, there is the possibility that there is insufficient air exchange in the atmosphere around the aquarium. In other words, the interior air is not being freshened enough. This problem has raised its head on occasion. The remedy is to pull in more fresh air, with extractor/intake fans for example.
Provided the seawater in the test bucket has not been contaminated in some way, it can be returned to the aquarium.