Low pH/kH in Water, Low = Bad? Options

[Hamad]

I did a water test and my nitrate level is less than 10. There is no nitrite or ammonia, so the water is perfect and I do around 50% water changes every 2-3 days. There's also good filteration and high aeration and heater set at 30C.

But to my shock, the PH/KH are very low and they probably were for awhile because I did not test PH/KH for months as I don't buffer or try to make any changes to it. KH is less than 3d and the PH less than 6.4.

I did a test to the tap water and the results are very much the same. Does this low (but stable) level of PH/KH could have a side affect on the fish?

I notice that my fish usually show stress (very low energy) even though I feed a healthy mixed diet [different pellets, bloodworms, spinach, etc...] also as I mentioned earlier water is great and temperature at 30c which means fish should be more active than this. Could the PH/KH have something to do with this?

[Sabine]

Hamad, I do it this way: after syphoning 2/3 of the water out I fill a jug with tank water, add the baking soda and the dechlorinator to the jug, and start my hose to refill the tank. I let the water run into the tank through the jug (let it over flow), so the soda gets into the tank already diluted and well distributed.
After filling up again I test, and add more soda if necessary. I don't add it daily: if the KH is raised to about 4-5 the pH will be stable. No good gained from adding a little bit every day, this may just cause pH swings which will be stressful for the fish.
Fish going through a pH crash will build up a thick slime coat, and will be hanging at the surface with their mouths.
Regards