What the ratio leaves out
Water takes different paths and may short-circuit between outlet and intake. Dead zones and high-speed zones can coexist, so a tank-wide average does not describe local motion. The ratio also says nothing about media area, bypass or how often solids are removed.
This is why universal targets should be treated as starting ranges rather than guarantees. The evidence on hydraulic loading shows that flow interacts with load, temperature and reactor design rather than acting alone.
Turn the number into a diagnostic
Measure under normal installed conditions and use a consistent net volume. Compare the same system after maintenance and near its next service. A changing ratio can reveal resistance; an unchanged ratio with worsening water quality directs attention elsewhere.
If animals struggle, redesign or diffuse the return before chasing a higher number. If waste settles, alter circulation geometry or remove it locally. Optimisation aims at outcomes, not the largest ratio.