Well the high pH won't be hurting your fish but the test strips are not very accurate as you say, are you sure everything else is fine?
How big is the system? Was it previously cycled up before you got fish?
I recommend getting an API master test kit so you can be sure your ammonia and nitrite are safe.
How big is the fish tank? How much grow beds (what type) or filtration do you have?
I wouldn't worry about the pH for now, first you need to make sure the system is able to keep fish alive and generally with time the pH will come down.
ha ha! can we use swimming pool pH stuff for the fish or is it toxic?
Two Jay said:
Frying pan, a little salt and pepper?
a pH of 7.8 isn't that extreme. If you feel you must adjust pH, I recommend you adjust the pH of your top up water before you use it in the system. Adjust the pH down and let it settle (make sure it stays stable at least overnight) before using it in the system. I do not recommend using any acids directly in a system since it is far too easy to over do it and cause pH bouncing which is bad for all creatures involved.
I have used pH down for for spas which was sulfuric acid before. I wouldn't recommend using any particular acid constantly though. Muratic acid (hydrochloric acid) has been used before too. But you might do better to collect some rain water to use for top ups. Problem with constantly using acid to adjust hard water is that you will liberate too much calcium into the water and that is likely to cause potassium lock out issues.
I may be a bit "Jonny Come Lately" with commenting on this posting but perhaps the issue here is simple too much too soon. You say that you had 11 sunfish in there since July. Chances are that the bacterial load has not been fully established especially if the water is cool (50-70 degrees). I would assume that only a month into the Nitrogen Cycle that you are in the second tier of Nitrification and showing Nitrite in any accurate testing. Accurate testing is key with new systems. I see this everyday with people establishing ponds, AP systems and aquariums.
Another factor may be that even if the system had completed the Nitrogen Cycle, they system was only used to the bioload of those sunfish. The population/bioload was increased 500%! The bacteria simply could not keep up with the amount of Ammonia produced by the fish. It is always best IMHO to add slowly once you are 100% certain that the Nitrogen Cycle is complete and you are consistantly testing 0 ammonia and 0 nitrite.
Thats my $0.10 ...... good luck!
I once had a similar problem with one dead fish a day or so. After ruling out ammonia, DO, and the other usual problems I started looking into the plants in the system. At the time I was growing very large kava plants in the system. The roots of kava are used as a medicinal herd but the stems and leaves are poisonous. The leaves and small branches were getting into the troughs and rotting. This was just enough poison getting into the water to slowly kill fish. Once I figured out the problem I removed the kava plants and the fish stopped dying.
Are you growing anything in the system that could be harming the fish??