SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/fish-ph-drop

What often happens to pH in an established aquarium between water changes?

It slowly drops because nitrification and waste produce acids.

Start playing free

  1. It slowly drops ✔
  2. It always rises
  3. It stays exactly the same

Why: Biological filtration produces acid as bacteria process ammonia and nitrite. Regular water changes add buffering and remove acids.

Huh, didn't know that: Once the buffer is used up, pH can drop fast, which is called old tank syndrome.

The dad joke
What did the pH say after falling? I am feeling a bit acid.
Like this card?
Play the game

References

Answer: It slowly drops. Biological filtration produces acid as bacteria process ammonia and nitrite. Regular water changes add buffering and remove acids.

  1. UF/IFAS Extension FA031: Ammonia in Aquatic Systems edis.ifas.ufl.edu/publication/FA031
    Nitrification is an aerobic (oxygen required) process that results in the production of carbon dioxide
    Checked 2026-10-11.
  2. UF/IFAS Extension FA031: Ammonia in Aquatic Systems edis.ifas.ufl.edu/publication/FA031
    reducing the pH of the water unless sufficient buffers (alkalinity) are present.
    Checked 2026-10-11.
  3. UF/IFAS Extension FA031: Ammonia in Aquatic Systems edis.ifas.ufl.edu/publication/FA031
    once alkalinity is depleted, pH of the water can drop significantly and quickly
    Checked 2026-10-11.

Think this answer is wrong? Tap "Challenge this answer" on the card in the game and tell us why.

Test yourself in the game

Fish & AquariumsWhy should you avoid replacing all the tank water at once?Fish & AquariumsWhich pH change is most dangerous for aquarium fish?Fish & AquariumsIs a small bowl less work than a bigger filtered aquarium?

That's stuff everyone should know.