Effective water change calculator...

[Mugwump]

Here's a handy tool to guide you on the the changes really needed to keep your tanks healthy for your fish. ...just change the numbers shown to reflect the tank size, water change amount, etc...and hit 'calculate'..

https://www.hamzasreef.com/Contents/Calculators/EffectiveWaterChange.php

[terrapins]

A definitely handy tool to know about.