Measure and add context
Record the baseline and check a renewed surface without a persistent film. Do not infer the aquarium's condition from a single sign. Record method, time, temperature and recent changes so the reading can be compared.
Then compare mixing between the bottom and surface with livestock needs and recent changes. Assess the trend alongside breathing, behaviour and species requirements.
Correct without destabilising
Adjust one variable and assess oxygen needs balanced with planted-tank carbon dioxide goals before continuing. Keep a reference so the result can be compared. Address exposure or cause first, then retest before stacking products or interventions.
A completely still surface restricts exchange even when water looks clear.