Agree with both the above, but will add a bit of my own feelings on the matter.
First ditch the salt additions. Salt (unless marine mix) will do nothing for you buffering, and in any case, the issues is elsewhere. Salt did not drop your pH, Old Tank Syndrome dropped your pH.
You have a year old tank with twelve total water changes of unspecified volumes. You have fish, which produce ammonia. Ammonia is oxidized to nitrite by bacteria. Nitrite is oxidized to nitrate by other bacteria. That nitrate stays unless you change your water sufficiently to remove it.
Step 1: Measure your nitrate in the tank. It should be below 40ppm nitrate at least. At best it should be at 10-20ppm. Do serial (daily) water partials until it is where it should be. Vacuum the gravel with every partial.
The nitrification process described of the bacteria oxidizing fish nitrogenous waste uses, burns up, the carbonate/bicarbonate buffering in the water. Each milliequavalent of nitrogen oxidized to nitrate from ammonia uses two milliequavalents of buffering - the units and measures are trivial to you, do not worry about them - just remember that when your tank's filters are working, the bacteria are using up your natural buffers while they are detoxifying the nitrogenous wastes.
Step 2: Measure the KH of your tap water and of your tank. They should be the same. If they are not, if the tank is lower, you have lost buffer and need to replace it. Regular, routine, preferably weekly, water changes will replenish the natural buffers in the tank with those in the tap water. Your GH in the tank is likely to be higher in the tank reflecting replaced evaporation over the last year.
Established tanks do not need a lot of testing when operating smoothly. They do need to be tested for nitrate at least monthly. If the nitrate is creeping upward, you need to increase the frequency and or volume of your water changes. If nitrate is stable and from 10-20ppm, you are doing just fine. If your tap water is low KH (KH 3 or KH <3), checking the KH at the same time is good also.