Hi Nellie,
You can get a water test kit at any pet store as well as online. It is important to get the "test tube" or "dropper" kits because the dip strips are horribly inaccurate and therefore a threat to your pets as well as a waste of money. The dropper tests may seem expensive at first but you can use them for a very long time and so in the middle-term they pay off. And they are very accurate.
In the meantime, just in case your tank is in the middle of a cycle and water quality is the problem, you can do 20-30% water changes daily. This may also help cool the water temporarily. The article Ray listed is full of good info on cooling methods. Me, I had the most success with closing windows and shades during the day until the evenig was cool, then opening everything through the night and early morning. In this way Iws able to cool the entire room to be quite chilly although it was summer outside.
If nothing else, you can put your axolotls into the fridge. This almost certainly will not improve their appetites but will slow their metabolisms so that they do not need as much "fuel" (food). Lots of axolotls "down under" spend a lovely summer vacation in Fridgeland each year. More info on fridging here:
click.
Good luck to you; please keep us posted.
-Eva