Question
Should I add salt to my water. I have 3 baby koi and 3 slider turtles in the same tank. Next summer I am building a pond in the backyard. Can I use solar salt for this purpose.

Answer
I suggest adding salt for both koi and turtles.  A tablespoon per 5 gallons is a conservative amount.  I use that in all my aquariums and turtle tank.  Salt deters bacteria, funguses, and parasites and helps with osmotic regulation.  For more on salt, see my page at http://www.fishpondinfo.com/fishcare/salt.htm
I'm not sure what "solar salt" is.  If it's just sodium chloride, then you can use it.

Good luck!