Ph Testing

We need 2 testers as it is difficult to make one bottle that will test from 6.0 all the way up to over 8 with any degree of accuracy. Back in my student days we used pH testers with a narrow range, choosing the one that was most appropriate.

My pH is like yours, I just call it 7.5. Knowing the pH to within +/- 0.1 is not that important. Yours is slightly alkaline, which is really all you need to know.
If you have have low KH and like to check every now and then that the pH has not dropped, the lower range one is the best to use as that will pick up a drop. But if you are using a substrate or decor you are worried will raise the pH, use the high range one to keep an eye on it.

The problem with the API ammonia tester is the light. In daylight, old fashioned incandescent bulbs and halogen bulbs, the reading is the most accurate. Under fluorescent tubes including energy saving bulbs, the reading can be off. We didn't have any led bulbs when I tested so I don't know about them. I could do with running a test to see.
A lot of people report they never see the pure yellow of zero. Testing a sample of distilled or RO water gives them a slight greenish tinge as well. As long as the free ammonia is in the safe region, a slight ammonia reading is acceptable. At your pH, and assuming your temp is 25C, 0.5ppm ammonia is safe.

Nitrite can give odd results if the level is off the top of the scale as the tester just can't cope with levels that high. It has been known to give a greeny blue shade after shaking and standing. The giveaway is if the drops turn purple when they sink to the bottom of the tube which then vanishes on shaking.
You can check to see if the level is off the top by diluting a sample of tank water with four parts tapwater - ie 1 part tank, 4 parts tap. This won't give a very accurate result as you need very good measuring equipment to do it properly, but if the result is a lot lower than pure tank water that indicated that the tank water is indeed off the top.

Nitrate tests are only ball park anyway. It is impossible to make a really accurate tester for use in the home. The best use of nitrate testers is for following a trend eg is it going up during a fishless cycle, are my water changes getting it lower than it was before.