It's not that the water there would be bad, but when a living being is in the water it produces wastes that are harmful for them. No matter how perfect the water is, it won't be perfect for long with an animal living in it. An axie produces wastes in the tank -> ammonia, which is highly toxic for them. Bacteria take the ammonia and produce nitrite, which is still very toxic. Another colony of bacteria takes the nitrite and produces nitrate, which is not as toxic but will be harmful in higher concentrations which is why you do regular water changes. Those are the reasons why you need to test the water. I never have to add anything to my water either, but I still test the tank water regularly to make sure the cycle is working properly - ammonia and nitrite 0, and nitrate under 40 ppm.
Edit. I went back to look at the picture of your axie and noticed the filaments in his gills are very short. Wrong water parameters affect them too. Now it might be because of other reasons, but wrong water parameters is one of the most common reasons. So I would definitely have the parameters checked (right before water change) or get a testing kit so you can test them yourself (Api freshwater master test kit is very popular).