it depends on how low the pH is and what fish you are keeping.
if the pH is 6.5 and you are keeping the gold killifish, don't worry about it.
if the pH is 5, then add some shells, limestone or dead coral skeleton/ rubble and monitor the pH over a few weeks.
if the pH is 6.5 and you are keeping the gold killifish, don't worry about it.
if the pH is 5, then add some shells, limestone or dead coral skeleton/ rubble and monitor the pH over a few weeks.