Now, what's the problem? Why are u adding salt? If there's no problem, no need to add salt. We're dealing with Discus here... Not some Marine Angel.

Salt doesn't evapourate/dissipate so u do not keep adding it. The only time salt is removed from the aquarium is thru water change. Considering that u're keeping discus and probably doing daily or every 2 days water change, u're going to find it a little tricky to keep the salt concentrations at the right level. My advice is if there's not problem, keep the salt in the kitchen.

However, if you do have to use Salt for treatment, I suggest u use 0.3ppm salt levels (0.3% salt concentration. ). Makes it easier that way... no guess work (my spoon bigger than yours!). Measure up your tank, and get it's volume in L (1L = 1kg). ) 0.3% of the weight of the water in the tank = to the weight of salt you should put in. At 0.3%(or ppm, same thing) salt, all problems salt can treat will be treated. This 0.3% is recommended for long term treatment... ie u can keep the fish in there for up to 2 weeks.

A salt dip.... now that's a dangerous thing to try. That uses 1% salt solution (ie pretty close to Sea water), and u put the fish in for 2 min (or until it goes on it's side.. whichever comes first.) This burns off any external parasites (doesn't work on flukes though)..... I don't suggest u do the salt dip.

There's no need to keep salt in the water long term. I don't.