My wife and I have several fish tanks, mostly freshwater and planted with live plants (impressive if I may brag a little). We also have one tank that is a brackish tank. The salinity is low in comparison to full saltwater. When we clean fish tanks we like to use the water for the plants as they love the nitrogen and trace minerals it contains. But we currently toss the brackish water because we're not sure what levels of salinity the plants can tolerate. We have a hydrometer to test water salinity, and it reads 12 PPT (parts per thousand?). Is that too high for the garden/lawn?
Any help would be great so we can know if we are able to save all that water. Thanks in advance!

I would toss the water on the compost pile rather than on the plants themselves.