Help with solution

Hi all, hope you can help

2 tanks with the same problem;
NO3 - 100-250 - too high
KH - 3 - too low
PH - 6.4 - OK
GH - <21 - too high
NO2 - 0 - OK

How do I raise KH and lower the NO3 and GH?
Water used is from a spring - tested to be excellent.
Little plants and natural rocks.
Just medicated for itch, but fish are still flicking a bit. One plec has died. Kribensis are eating fine, Goramis seem fine. Last plec not eating.
No problems in these tanks for last 4 years.

You should test for ammonia, which is more toxic than nitrite. Both compounds are caused by a biological load that is too large for the beneficial bacteria to handle. More live plants, a little less food, and regular partial water changes should take care of the problem. You might also get the spring water checked; with all the pollution in the environment, the water could be tainted.

Since most popular aquarium fish have been just that for many generations, they're pretty well adapted to a variety of water conditions. A sudden change in any water parameter, however, will shock them.