Changing water vs. adding water

[Hank Paloci]

I have a system consisting of about 225 gallons of fish tank and 36 square feet of grow beds (2 3x6' by 8 inches high). I keep my fish in the garage and pipe the water in and out to the grow beds behind the garage on the south side of my house.

Due to evaporation and use by the plants, I find that I have to add perhaps 20 gallons of water per week to top off the system -- more in hot weather.

Usually, I'll drain off some water (5-10 gallons a week) when I'm adding in and use it as fertilizer for the grounded plants around the house, like my orange tree. So, the system is getting maybe 10-15% new water a week, but due to evaporation I figure it's retaining comparatively more dissolved solids.

At some point, I feel like all the water I add in to compensate for evaporation is going to mess up my water chemistry. Tap water in the spring in particular here in SoCal is going to be full of mountain runoff, which contains lots of minerals. At some point, my tanks will get too salty. Tilapia can tolerate this, but there are limits.

I'm wondering what, if anything, you to about this. I know it's hard to quantify solutions because everyone's water is different, but any ideas for how you deal with this (if at all) are appreciated.