I use water from my water softener inside the house for my fish tanks. It is softened with salt. Its a pretty small amount, but with evaporation I'm afraid the salinity will creep up and hurt the plants. Is there anything I can do besides switching water? The only source of unsoftened water is by the spigot out front. I don't want to be carrying 30 pound buckets all over my yard, house, second floor, room and bathroom. The gH is also drastically different. The softened water is about 4 and the hard water is way above 300. I have black neon tetras, otos and soon a honey gourami in the tank. I don't think they'd like that water.

Do you have/can you get a refractometer or something to measure the salinity of the water? Since salt does not evaporate and cannot be removed you will just have to keep a check on the level and keep it appropriate. What the right level is I don't know, maybe someone will come along soon with another idea.