6.0 is far too low. Although axolotls can tolerate a wide pH range of 6.5-8.00, their preference is toward the higher end of the scale.
You're right not to use chemical pH uppers and downers as these 'quick fix remedies' can cause wild fluctuations in pH, which is even more dangerous to the axolotls than leaving the water at an imperfect pH level. Also, most chemicals do not agree with axolotls, it's best to avoid adding unnecessary chemicals (such as pH upper and downer) to the water when it can be avoided.
Now for the good news!
There are quite a few safe natural methods at your disposal, such as shell grit, coral sand, and limestone, that you can use to raise and then maintain your pH at a consistent acceptable level for the axolotls (which is what you wanted). Another beauty of these is that the pH level adjustment is a gradual process that will not upset the axolotls.
You can buy shell grit from most pet stores.
The leg of a pantyhose makes a perfect sack for the shell grit.
- Cut a good length off and add some shell grit into the "sack" (I would add about a *1/2 to 1 cup of shell grit at a time)
- knot off the end (not too tight, as you may need to add more shell grit later)
- give it a shake in a bucket of the tank water to clean the grit off (it's quite dusty! If not cleaned first, it will cloud up the tank!)
- and then add the stocking to the tank.
Let it sit in the tank for a few days. The change will be very gradual, so test the water every few days, and if there is no difference, add another 1/2 to 1 cupful to the stocking and repeat until the pH is where you want it.
You can use the same method with coral sand.
* 1/2 cup at a time if you have a smallish tank (40-60 liters/10 gallons), 1 cup at a time if the tank is larger.