In the weeks before and while I had fish in my tank my API test kit showed the PH at 8.0. A LFS in Houston and my local LFS also tested my water with the API test kit and it tested at 8.0.
I went yesterday, with a water sample, to a third LFS and he tested my water with a Salifert test kit. I had tested the same sample with the API kit and it showed 8.0. His test showed that it was just about 7.0. When I related the story of how my fish died, he said that it was the PH being so low. I can understand this if the PH is 7.0 but is there any symptom that the fish would show before dying that would prove this? I had gone there to buy some snails to clean my tank but he refused to sell them to me. He said that until I fix the PH I would just kill anything that I put in there. He told me what to buy to raise the PH, and although his store sold it, he told me that it would be less expensive on the internet.
Any help would be appreciated.

So, you used a liquid test kit and it tested ok, another tested it ok, then with the third test, it comes up bad. I don't buy it. Ask him to test it again with another test kit. Your kit isn't all that old. I would take your reading and the other LFS as 2 good tests.
Seawater
The pH of seawater ranges between 7.4 and 8.4 and averages about 7.6. Aqueous CO2 in seawater combines with carbonic acid (H2CO3) present in seawater, giving off a bicarbonate ion (HCO3) and a carbonate ion CO3 randomly which helps to maintain the pH level.

Salt Water Aquarium
If a pH of 7 is neutral, then a salt water aquarium should have a pH of between 7.6 and 8.4. If the salt water aquarium is recreating a reef environment, then the pH level should be between 8.0 and 8.3.

The “How To” Guide to Reef Aquarium Chemistry for Beginners Part 3: pH by Randy Holmes-Farley - Reefkeeping.com

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Low pH means high carbon dioxide.

Fish symptoms could be heavy breathing, slow moving, a slow deteriation which could happen over a few days or even few weeks.

If you add macro algaes (in a refugium) and get them thriving and growing you will see an almost immediate (day or two) increase in pH. And your fish will really appreciate having more of their carbon dioxide removed and replaced with oxygen.


my .02

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden

I just bought Salifert test kits for PH, calcium, phosphorus, and alkalinity, and a 2 pound box of Kent superbuffer/dkh. I also bought a larger tube for my overflow box to increase the flow from the sump. I am also going to get at least one more powerhead, and Salifert Ammonia, nitrite and nitrate tests.
This LFS guy asked me to bring him a sample of my well water for testing for everything, a sample of mixed saltwater, and another sample from my display tank. One of the things his is going to test on my well water is total solids. That might reveal the source of the silicates that the diatomes are thriving on. He also said that I might need a RO/DI unit to treat my well water with to get rid of the silicates. I live on the gulf coast. My dirt is very sandy. I can believe silicates in my water.

That fits. Looks like my fish suffocated. I am taking steps to correct the problem now. I will research on how to setup a refugium. This may be a bridge between FOWLR which I currently have, and a reef setup. I have a 29 gallon sump.

FWIW refugiums do not have to be big complicated things with pumps and what not.

All you have to do is partition a small part of the tank so you have a macro area and a fish/coral area. I just crammed in a 1/4" plastic grid lightiing diffuser 3" in front of the back glass on my 55g. And added 2 utility lights behind the tank pointing forward. Macros and pods thrived behind the plastic grid, fish and corals in front. Nitrates dropped to unmeasurable levels in 3 weeks with phosphates dropping there a few weeks later. pH rose from 7.4 to 8.1 in 24 hours then 8.4-8.8 a week later and stayed there for years.

I yellow tang who had already developed white spots recovered and doubled/trippled in size in a year and lived for years after that.

Good luck

my .02

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden

Update: I received the Salifert test kits today Several things I discovered about my API test kits. first, the little test tubes with the 5 ml mark. The mark is way off. Get yourself a syringe that measures in ML and use that to fill the test tubes. It will make them more accurate.
I used a PH booster. After 4 treatments, the API says the PH is still about 8.0. The Salifert test kit also says it is just over 8.0. Thing is, API tests always said it was 8.0
Killed seven fish because the test kit wasn't accurate.

Other test results

Phosphorus 0PPM
Calcium 440
hardness 15.7 dKH
Alkalinity 5.59 meq/L

The hardness and alkalinity are high? What would make the water hard? Should I test my water supply? What am I looking for. I do know there is no lime or iron in my water. those things the swimming pool company tests for.