Steveo, you did quarantine him but for a very short time, rather than what would normally be the 30 day quarantine period (wasn't it 10 days, but then like most people that haven't been advised properly, you didn't know).
As for ideal fridge temperature, minimum of 5C. Check by placing a thermometer in a glass of water and placing in the fridge for 15-20minutes then check.
You can use a vegie bin or icecream container with lid when fridging.
Alternative to tanks, if you have any, big plastic storage bins on wheels, a plastic bucket or spare vegetable bin if you have one. You'll need to change the water daily on temporary tanks though. If using a bucket, make sure you place something over the top so he doesn't jump, or if you have a cat so it doesn't hurt your axie.
Feed the smaller one once or twice a day, try and feed earthworms if you have them. Clean up any uneaten food within a few hours.