before you get any more I would make sure you have a cycled tank waiting for them. The stress of arriving and then leaving the pet store, being bagged/netted, and then introduced into a new system with growing levels of ammonia can be quite a lot for an ADF to handle. Stress lowers the immune system so they are coming to you from the store already a bit under the weather. Even being shipped by a private breeder will expose them to stresses (no one likes being moved whether it's in a bag or a box!).

Not sure what size tank you have but I would aim for a 10 gallon or a 20 gallon long as a great starting point - you could have a great colony in either of these tanks and the extra water volume helps with water stability.