na2 and female guppy problem

Question
I have a 5 gallon tank that I thought would be cute to put  goldfish in for my son, soon to find out how much of a dirty fish they are. I gave them away to a pond and hand my water tested. Results were NA2. I had one Strawberry Tetra that has managed to survive the NA2. So with the advice of the fish store, I replaced the water 100, cleaned out the tank and marbles with vinegar and  thoroughly rinsed with the hottest water and replaced the filter pieces. I was advised by the store I could put in 2 female guppies, 2 male guppies  and 1 albino catfish and 2 golden mystery snails. I have had the fish in short of 2 weeks and yesterday, I noticed the one female guppy swimming be herself, not moving from the spot and close to the surface. The fish doesn't seem like it will survive. How do I lower the NA level besides reducing what I feed? I have also done a couple 10 percent water change and allowed the new water to stand over night and have put in the Tap Conditioner as well, per instructions.
What am I doing wrong?

Answer
Who told you that you could put that many fish in there?

In my opinion, no fish should be kept in a tank under 10 gallons except species of dario dario and betta splendens, and that is with very frequent water changes.

I would return all the fish you have, and exchange it for a single male better.

40% water changes twice a week.

To lower the NA you need to cycle your tank. Type in Aquarim Cycling on google and check out some of the results.

-Nick