How Much Water Conditioner

[allh2o]

New member, I did a sheach but couldn't find. (User error)
I have a 20g tall aquarium. My question is when I clean my tank I take 25% water out of the tank.
How much water conditioner should I put back in? 25% of the tank or 100%.

 

[fluttermoth]

Just treat for the 25% you've taken out.

 

[Byron]

Agree.  It is always best for the fish to not overdose any chemical substance.