As has been mentioned, fish tank water is ideal for all plants, indoor and outdoor. When I had the house with a garden, I had a couple of large garbage-type tubs outside the back door and I filled these on water change day, then used the water over the next few days to water the garden shrubs.
I also like to pour a bucket or two down the toilet each week. These low-flow toilets can clog up especially if you are on the ground level, but this always kept mine running free. I also do the same in the sink in which |I used to rinse sand/gravel, just to keep anything from building up over time. In 18 years it never did.
I would use it on my house plants but my husband would object. I should never have used the word bacteria in his presence, he thinks that we'll both come down with some awful disease if we touch it by accident. However, what he doesn't see is me emptying the water over the garden plants in summer
We're approaching Halloween, time to scare him then. Tell him there are protozoans, spirozoans, fungus, bacteria, viruses, parasitic worms that burrow into the fish, tapeworms, thread/ round worms and even Tuberculosis in aquariums. And if he's not careful, you will splash him with fish tank water