You can put the conditioner right in the tank when adding the water from the hose (Python?) but I disagree with meegosh on the amount of conditioner to add.
I suggest that if you are putting water directly into the tank from a hose you should treat the entire tank, not just the amount you are adding.
That is true for Seachem Prime.
Seachem. Prime
"Directions
Use 1 capful (5 mL) for each 200 L (50 gallons*) of new water. This removes approximately 0.8 mg/L ammonia, 1.2 mg/L chloramine, or 3.3 mg/L chlorine. For smaller doses, please note each cap thread is approx. 1 mL. May be added to aquarium directly, but better if added to new water first. If adding directly to aquarium, base dose on aquarium volume. Sulfur odor is normal. For exceptionally high chloramine concentrations, a double dose may be used safely. To detoxify nitrite in an emergency, up to 5 times normal dose may be used. If temperature is > 30 °C (86 °F) and chlorine or ammonia levels are low, use a half dose."
Also true for API (Aquarium pharmaceuticals) products
Tap Water Conditionerâ„¢ - API
I called the help desk at API (1-800-847-0659) and they also said to dose for the entire volume of the tank, for all their products, when using a Python.
Apparently it is not on the product labeling because of the Python being a competing companies product. He also said it would be very difficult to overdose their conditioning products. { I asked what if I dropped the whole open bottle in the tank and he said he would do a 50% water change but it should not affect the fish.}
For other water conditioners best to check the manufacturers directions to be sure.
