goldfish sitting on the bottom

In the last two days I have lost 2 goldfish and another 3 are sitting on the bottom of the tank. Two are still feeding.

My tank is 160litres and about 4 years old. The filter was replaced about 6-7 months ago and is an aquapro 130F. I clear one of the two sponges each month (on rotation) using tank water.

PH is 7.2 (liquid tester)
Ammonia 0
Nitrate 0
KH and GH is about 12

I have goldfish, Oranda's and black guppies (?)

The sick fish do not seem to have any fungus, discolouration or bulging eyes or anything else obvious wrong with them.

I have had a problem with the GH in the past and this weekend bought a water softner pillow which I just sat on the bottom of the tank for a couple of days. I have now removed it.

When the first fish started rolling on the bottom I added 3 tablespoons of aquarium salt. I do not know what the salt level was before I added it.

I do a water change on a weekly basis adding about 20litres of natural spring water (because the water is so hard where I live).

I am not sure what else I can tell you but my fish are dropping like flies.

Please help me!!

I have an airstone, suppose I could try that. Medications will have to wait until tomorrow though, sadly another one has dropped to the bottom. Probably won't have any left by the weekend