creamsicle lyretail behavior

Question
QUESTION: I am new to having fish and got a 20 gallon tank recently for my children.  We have had it for about 2 weeks now and am having a bit of a hard time with one fish in particular.  A creamscile lyretail.  The first one looked healthy then a day later after getting it home I noticed some white puffy stuff on one eye, the next day it was dying.  Brought it back to the store and got a replacement.  Water tested ok.  Now the replacement is not doing well....lying around looking lethargic but seemed ok this morning.  I brought a silver lyretail a few days ago and that seemed far more subdued when I got it home but has recently perked up.  So now the silver one looks ok and the creamsicle looks, well like its dying.  What is normal behavior for these fish?  I am afraid I may have missed the warning signs.  Eating regularly after you bring them home or is it ok for them to not eat sometimes?  Energy level?  What area should they be more active in in the tank?  They have both done a lot of laying on the bottom and a not moving around much.  Their beautiful fins are not unfurled and are sort of folded up and made minimal.  What is going on with them?  I am afraid I am going to lose the creamsicle and the silver one may follow.

ANSWER: Hi Laura,
Lyretails should be actively swimming and not laying on the floor and barely moving.  It is common for fish to be non-active after giving them a new home to adapt to.  But they should adapt within 1-2 days and be eating by then.  She could be stressed out.  It takes a long time for fish to die of stress.  Fish that are stressed will usually start to hide, and if that doesnt help they will eventually start to die.  If this is what your fish is doing, then cut the lights if you havent, this will aid in hiding and decrease stress and also add Aquarium Salt.  Its not the same as Sea salt for saltwater fish.  Aaquarium salt is more like electrolytes that live-bearers (Lyretail is a livebearer) appreciate and will promote healing and stress.  If its not stressed and your water quality is good, it could be some kind of virus that is going around the tanks in pet stores.  Or possibly due to some bacteria.  Disease in fish is out of my expertise but I do know that Maracyn is a great antibiotic to treat bacteria.
-Matt-

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QUESTION: Well this isnt a question, but I thought the follow up would be relevant.  Thanks so much for your help.  I did go and get the aquarium salts and discovered that there was a fungus that I couldnt see present.  It appeared and was very hard to see but I caught it on the fin of the creamsicle fish.  So I found out I had to treat the tank and I did that and it seems all of the fish are responding well, with the exception of the creamsicle that just passed while I was out of town briefly.  But I think this will prevent any future fish from getting sick.  Thanks so much for your assistance.  
Laura

Answer
Hi agian Laura,
Yes no problem.  Remember to keep good water quality at all times by doing water changes, turn lights off for at least 8 hrs to help the fish "rest+sleep" to reduce stress, and provide hiding places.  Good hygiene of the tank will allow the fish the build a strong immune system to fight agianst dieases.  Bacteria infections are rare in a well maintained mature aquarium.
-Matt-