Back from work ... Here is why not to use pH down.
1. Axolotls are very tolerant of pH, so there is almost never a reason to change your tap water pH to suit your axolotl. The exception is if your pH is too low, it might need to be raised. But even then, don't use the commercial pH adjusters without researching what is in them.
2. Axolotls (and amphibians in general) are sensitive to some chemicals that don't harm fish. Most aquarium products are tested on fish, not amphibians. pH changes/stabilizers are seldom labeled as to what they contain, so you can't verify they are safe.
3. pH Down is a very strong acid. It is easy to overdose and burn your axolotl. They don't have scales to protect their skin.
4. High pH tap water usually has a lot of buffering capability. That means, when you add the pH Down, the pH will drop at first, but then will rise right back to where it started over the next several hours. So then you have to add more - the pH drops, then rises again. You have to do this a whole bunch of times before the buffer breaks. And your poor axolotl is living in this tank while you do this acid-to-alkali-to-acid-to-alkali yo-yo, which is very stressful even if you don't burn him.
If you really want to adjust the pH of the water for your axolotl, it is best to get it totally adjusted and stabilized in a separate mixing tank, and then do water changes with water from the mixing tank. But remember, with high pH, it takes a while to get the buffer broken.
Back to the other issues ...
In that size range, your axolotl could be as young as 4 month and is probably not more than 7 months, So, it is still a juvenile.
Like Skudo asks, do you understand the cycle? Do you have a test kit that measures ammonia, nitrite, nitrate, and pH? What are the readings?
It sounds like you may be very new to aquarium keeping. If so, it is likely that you have an uncycled tank, and that the ammonia is starting to rise to toxic levels. This will cause all sorts of problems (ammonia burns, for example). If this is the case, you will need to be doing some large water changes to keep the ammonia (and nitrite when that starts rising) to non-toxic levels while the tank cycles.
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