Hello and thank you for your question.
Without more information I need to assume a few things, first that this is a freshwater tank and second there is nothing in the tank that is causing the pH to rise (such as sea shells or pieces of corals). Many places have a higher pH associated with their tap water and this may be why your pH is at 7.8.
To lower the pH there are a couple of different methods you could use. One way would be do use some distilled water mixed with your tap water. Distilled water has a lower pH and will help to balance out the pH prior to the water being put in the tank.
There are many different types of additives supplements that you could use as well. I have linked a few below for you:
Seachem Lab Neutral Regulator
API pH Down
Seachem Lab Acid Buffer
If there is anything else we can do for you please let us know.
Marine Depot Customer Service