Fish Tank Water?

I have a 75 gallon fish tank that I would like to use my weekly water changes to supply water to my two 4 gallon reservoirs. The ppm read 1600 on average at a 7.2 Ph. I have a friend who only uses fish pond water for his outdoor grows.

Question is will PhDown effect the usefulness of the fish tank water?

I'd be more worried about some crazy algae outbreak using water from my fish tank. Also at 1600ppm the water seems a weeeee bit too high on the EC levels there.

the plants love it i've used fishtank water as the base then added canna coco nutes to it during veg + ph down and they all had lovely deep green to them. i did that once a week when changing the water

i use fishtank water, im in soil.