I’ve kept many, many types of fish – including neons, and currently cardinals, in my standard tap water, which has faithfully sat at PH 8.3 for years. My fish are healthy and long lived, and disease is a non-issue for me.

If you start messing around with the water it tends to become less stable, and a fluctuating PH is much more stressful to the fish than a PH that is too high or too low. This instability is especially risky if you go the quick and easy method and use those lousy bottled solutions from the pet store, like PH Down. If you really want to lower it a bit, just mix R/O or distilled water with your tap water to achieve desired results, and top off evaporated water with R/O or distilled, as mentioned above. But really, un less your GH is very very high I wouldn’t worry.

Neon tetras like water that is fairly soft (dH around 15-20) and a pH around 6.5. They do not do well in water that is hard or the pH above about 7. So it will be best to use bottled water for the initial set up of the tank and use only distilled water to replenish the water that evaporates. You’ll need to check the hardness of the bottled water, as some bottled water can be fairly hard. And, as you know, you’ll need to check the pH. I use the Quick Dip 6in1 test strips. They check pH, total hardness, total alkalinity, nitrite, nitrate, and chlorine. I carefully cut them in half, lengthwise, w/ a pair of scissors to reduce the cost per test.

Of course there are other options. Aquarium supply stores have chemicals to change the pH. This will require a combination of pH buffers and pH adjusters, since you will have to watch both the pH and the Total Alkalinity. Also available are resins that will lower the dH (water hardness). I personally do not like these options as they require too much tinkering w/ the water chemestry.

Neon tetras do best if kept in large groups – 6 to 8 minimum. Depending on you tank size and the number of other fish, 1 to 2 doz is better as they like to huddle, and the display is prettier too. Although neons are easily kept, you can expect a 10% attrition w/ new fish. So buy a few more than you wish to have as the minimum number.