Some aquarists recommend adding minute amounts of table salt to the aquarium to affect the pH of the water.
But since I have a saltwater aquarium, I never really had the need to explore that.
Of course, water from saltwater aquariums should not be used to water orchids anyway

Some considerations for freshwater aquariums:
1. Some of the chlorine/chloramine "removers" do not actually remove the chlorine and heavy metals from the water. They simply bond with those substances to make them inert and thus pass through the fish' body harmlessly.
However, would there be a difference in chlorine/metal content from your aquarium water and the fresh (tap/RO/well) water that you would have used to water your plants anyway?
If the bonded chlorine/chloramine is harmless for the fish, would it be harmless to the plant?

2. The organic waste contained in the old aquarium water might actually feed the plants. Perhaps along the same lines as compost tea but to a lesser degree.
I just do not know personally if the puzzle pieces line up. Are the nutrients contained in the old aquarium water the same nutrients required by the plants?