A number of factors can affect the pH in an aquarium and lower it.

The first factor is the source water. If your tap water has a low pH and you fill your tank with it, your tank water will then also have a low pH. The pH of tap water can change occasionally so if you experience a drop in pH in your tank and you’ve just done a water change, check the pH of the tap water too.

If the tap water has a lower pH than the tank water, it’s the tap water that’s lowering your pH.

If you use Reverse Osmosis water for water changes, that has a low pH too, so if you keep using it your pH will naturally go down and down. Add a buffer or remineraliser to RO water to prevent this from happening. That way you get all the benefits of RO like low phosphate without the low pH. If you use rainwater in your aquarium, that will lower pH too. 

Next is if you have any materials in the tank that acidify the water and lower pH. This could be bogwood, Catappa leaves (for shrimp,) peat, blackwater extract, some planting soils, or any other botanicals like oak leaves. Many fish species like tetras, rasboras, Discus and L-number catfish like and appreciate a low pH. But if you have hard water-loving livebearers and Rift lake cichlids remove any of the above materials to prevent further acidification of the water and lowering of the pH. 

Lastly, aquarium water can itself acidify over time. If you didn’t change the water for six months you would expect the pH to be lower than the day the tank was filled. This is because a build-up of fish faeces acidifies the water, as well as dead plant leaves and even biological processes going on inside the filter. 

How to fix low pH

If your tapwater has a higher pH than your tank, a series of partial water changes will raise the pH. If you use RO water combine it with a pH buffer to set the pH at what you want. Buffers can also be mixed into tank water overtime to raise pH without changing the water. Although a water change is always best.      

If you have fish that like a high pH, add calcareous materials to the tank, like coral sand, coral gravel, tufa rock, ocean rock or limestone. Monitor the pH of aquarium water with a pH test kit.