While a pH of 8 is higher than ideal for many soft water fish such as bettas, maintaining a stable pH is preferable to the instability caused by attempts to lower it. Algae typically do not cause an increase in pH, so it's important to investigate the underlying cause.
To accurately assess the source water pH, fill a glass with water and let it sit for 24 hours before testing. If the aquarium's pH is higher than the source, something within the tank is likely causing the rise.
White gravel may contain calcium, which can leach into the water and affect pH levels. Check the packaging to see if the gravel is specified as inert. For a practical test, place some gravel in one container of water and none in another, then compare the pH after a week.
Additionally, consider whether there are other rocks or stones in the tank that could be influencing the pH.
Knowing the carbonate hardness (KH) and general hardness (GH) of both the tank and source water can also provide insights into water chemistry.
Lastly, it's useful to know how frequently and in what volume you perform water changes in the aquarium.
