Fill buckets and bowls with water now! Let the water sit for 12 hours - should be longer but you don't have the time - are they tropical, cold water or marine? Do what needs to be done to get the water ready for the type of fish you have and seperate the new ones from the old ones and any oder fish that don't look so good from anyone else as soon as you can once the chlorine and temperature and salt have been sorted. Go buy small plastic tanks if you have to in order to do this - and think about sympathy buying...if you buy them, for whatever reason, you are increasing demand for them and so more fish end up in a tank in a store being sold as reptile food. Healthy fish that can be sold at a profit to fishkeepers will not end up in the reptile food tank. So, it follows that there is something not quite right with those fish that are being sold as reptile food - if you buy them and put them with your pet fish straight away it is VERY likely that you will have problems.
Seperate the fish as best you can, be prepared for more loses and look at this as a lesson well learned. We have all done it at some time or another and the lesson is....don't create a market for ill bred, poor quality animals by buying them. It is incredibly hard to leave them where they are when they are suffering and in danger but it simply encourages more poor, prolific breeding of genetically unsound or diseased animals and fish.

Cruel to be kind I'm afraid.