Haha, I had the exact same worries when I started using my Python water changer. Then I saw my LFS using it to entirely change out their tank water in every tank and only add dechlorinator after they had changed the water in every tank. Depending on your tap water supply, some don't even use dechlorinators in planted tanks (I don't).
There's no doubt that the effect of the dechlorinator is reduced when dilluted into a larger body of water, but on the plus side, the effects of the chlorine are also greatly diluted in the larger body of water in your tank.
Go ahead and do your water change and then add the appropriate amount of dechlorinating agent right to the water afterwards. I think your fish will be fine.
- Sam P -
plantedtanker in limbo - all tanks currently in storage