When I do water changes, I fill 5 gallon buckets and add the water conditioner. I let the buckets in my patio while I am taking water out from the tank, cleaning the decorations and so on. I always do this before noon, when it is not too hot. I have never had an issue doing it like that.
I put the water in using a plastic jar with a handle.
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When the water level is low, I put the jar against the glass and the water slides down slowly creating minimal effects on the
sand. Once the water level is higher, I just insert the jar and turn slowly, that was there is only a current at the top of the water level.
Hope this helps.