Usually minimum quarantine period is 30 days, and then you try and stagger when you put them in, ie add one one week, then the next the following week, to allow the tank to adjust to the additions (ie waste/feeding).
the other thing you need to test for is nitrite and nitrates.
When was the last time you tested your tankwater? What was the ammonia reading? If it was anything over 0, then you need to do frequent partial (20%) waterchanges to bring it down.
You can rule out any other possible problems by testing the water, or taking a sample to the petshop for them to test for ammonia, nitrites and nitrates; and keeping watch over the next few weeks (namely due to temperature) to all your axolotls. If you do take sample to petshop, make sure they write the figures down and don't buy anything (chemicals or whatever) they suggest to fix your tankwater. Frequent partial waterchanges will usually keep toxins low enough in the water for your axies to reside in.
Other methods for cooling are :
http://www.caudata.org/cc/articles/cooling.shtml
Alternatively, what a lot of owners do who can't maintain cool temperatures, move tanks and/or afford chillers, they get a 2nd hand fridge and fridge them for the summer.
