Under normal conditions, the pH in your aquarium will vary a little over time. Adding make-up water and performing partial water changes alter the pH to some extent. Hard water typically yields an alkaline condition due to carbon dioxide reacting with carbonates and bicarbonates in the water. Day by day, however, excrement and other sediments plus respiration by fish and plants dictate a tank's pH. Good housekeeping helps maintain a more stable pH. Your fish will appreciate that. Again, research may help you avoid tragedy.
Some species require a particular range of acidity or alkalinity.
They won't thrive outside that range. You may easily test the water with inexpensive litmus strips or with a pH test kit available at your pet store. Then, if needed, you can adjust the pH with chemicals also from the pet store. Just be sure to follow the instructions carefully. Often, good aeration and filtration, together with good housekeeping, will eliminate the need for any chemical adjustment of your tank's pH. Dissolved Oxygen Just like you and I, our fish need gaseous oxygen to live.