Hi, we have a 30,000 swim pond with about 150 goldfish in it. Been established since June 2016. Last week we dropped the water level down at least 1/3, probably a bit more than that so we could mortar some rocks. We have a 3 foot wide shelf that is about 8" deep and we dropped the water below that so we could concrete a rock wall that borders this shelf. We rinsed the rocks a lot and used a pond vac to suck up as much of the water as we could before it ran off into the pond. We kept the waterfall running that has a 10,000 ph pump but the other two smaller pumps we had to turn off due to the water level. After several days, the mortar was dry so we filled the pond back up with local city tap water. that is what was used to fill the pond to begin with but we do notice at times that there is more chlorine smell in the water than other times. I did not have any dechlorinator on hand at the time either. So within another day or so we notice the goldfish who usually gather in a large swell when they see us are now . . not to be seen easily. most are scattered around, staying near the bottom and seem very lethargic. I can just reach down and pick one up. Then we notice we got some dead bodies around different plants, etc. Physically they appear fine, other than being dead. I test the ph with my pond master test kit and the ph card says 7.5 which is a little higher than usual but still normal range. I thought if the lime in the concrete bothered the water it would make the ph spike. I tested ammonia, nitrates, nitrites too and all were well within normal range. So being a saturday, we took a water sample over to our local aquaria place (not petsmart) and they tested our water and said our ph was very LOW (which was curious because my test said the ph was 7.5, still within good dates and just 5 drops in the tube so nothing to really mess up) and advised us to put in pond salt . .half gallon when we got home, another half at bedtime, and same thing tomorrow, and then next morning. We also had them test for chlorine and they said it was at 0. (I had read that chlorine dissipates after 24 hours . . ) but we also bought dechlorinator and put that in the pond when we got home. So we did the salt. We also made a salt bath and would catch any fish that seemed to be struggling and put them in a salt bath for a bit and then back into the pond. They would seem to be gasping a little for air and would swim out of my hand but then would roll onto their sides at the top. So two days later, still finding dead fish every day. yesterday got another 6 out of there that were all about 4-5 inches long and several babies 1 1/2 -2 " long. The ones in there left are not really interested in eating, staying near the bottom, seem to be having trouble breathing. I've checked the ph with my kit again and it came out lower than 7.5, I believe at about 7.0 but I don't know if my kit is accurate because of the test at the aquaria store. So now we just go out every day and collect bodies . . .very sad. Any suggestions from others out there?