Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Yes, you can add it even without a water change, but I have had better results with a water change.
To reduce stress during a water change do a more complex water change that involves filling the tank as you are draining it. I have done some pretty big water changes that way, and the fish just keep on swimming around, never hiding.
Prep at least 2 tankfuls of water in a garbage can and start siphoning out of the tank. When the water level drops just an inch or two start pumping from the garbage can.
Keep the inlet from the garbage can high up in the tank, and aim it horizontally.
Siphon from the bottom, vacuuming the substrate and among the plants the best you can.
Ultimately the new water and the old do get mixed, so you are removing some of the old and some of the new, and I understand the math shows that this method results in only about half the water change compared to a drain first then refill method. Still, by vacuuming as close to the floor of the tank as you can you are removing more debris than if you were just letting the siphon hang over the edge, so hopefully removing whatever is contributing to the stress of the fish.
Doing water tests before and after has shown me that (within the accuracy of the tests) I can get something close to 75% water change when I use about 2 tanks worth of new water.