I have a 30-gallon (114- liter) aquarium that houses three fancy goldfish and one Siamese fighting fish. The aquarium has been set up for a little over two years. The pH is 6.1, which I know is a low reading. I have read that this can be harmful to the fish and the biological (undergravel) filter in my aquarium. My question is: How do I raise the pH?
The pH of your aquarium is right at the lower survival boundary for both the goldfish and the betta. As you say, it also inhibits the proper functioning of your undergravel biological filter. Although the fish have evidently adapted to the low pH, this is not a healthy situation.
In my experience, goldfish held for long periods of time in water with a pH below 6.5 become very susceptible to bacterial infection. The rate of healing for wounds is also greatly reduced.
The source of the acidification of your aquarium water is not clear from your letter. Have you tested the tap water pH? If the tap water has a substantially higher pH, more frequent partial water changes will take care of the problem. Keep in mind that any properly operating biological filter — whether in an aquarium or a pond — will naturally acidify the recycled water over time.
If your tap water is acidic, or very low in alkalinity, then I suggest that you begin a gradual process of raising the aquarium water pH with the addition of ordinary sodium bicarbonate. I use Arm & Hammer baking soda, which is available at any supermarket. Add about one-half of 1/8th of a teaspoon to a cup of aquarium water, stir and then pour it into the aquarium. Wait one hour and measure the pH. If it has increased by 0.2 units, stop for now. Continue the same procedure for as many days as it takes to raise the pH to 7.0. to 7.2.
As I cautioned in answering the previous question, sudden changes in pH are more dangerous to your fish than low pH water. Go slowly. The fish have survived in this water for two years, so they will last another week while you gradually change the pH. Once the pH is where it should be, you will have to monitor the pH every other day and use either water changes or sodium bicarbonate to maintain a steady pH of about 7.0.