sick/dying Balloon Molly

Question
My Balloon-bellied Molly has been laying on the gravel for 2 days now.  She seems to be lethargic and her gills are moving quite rapidly, although, she herself is quite listless.  
This fish is housed in a 6-week-old 45 gallon tank and her tankmates include 1 guppy, 5 neon tetras, 1 black molly, a peppered cory, a dwarf frog, 2 hatchetfish, 3 guppy fry in a floating breeder box. I also have 3 different types of aquatic plants that have been in for several weeks.
This week, I have lost a koi and a black molly.  Last week, a male guppy died.    I did a 25% water change at the 4 week mark.  Do you think this could still be a 'new tank syndrome' kind of problem?  I added the fish gradually, a few at a time, every other week or so.
 Is there anything I can do for my Balloon Molly?  I don't want another fatality already if it can be avoided.

 I did test the water using a 5 in 1 test:  Nitrate:  20, Nitrite 0, Total Hardness:  50 ppm (moderate), ppm Total alkalinity/buffering capacity:  80, pH 7.0.
Diet is color enhancing flakes and freeze-dried bloodworms.
Any ideas?


Answer
Hello "Newbie",                            :)
It sounds like you do still have new tank syndrome. Many people do not wait long enough for fish. They think there tank is acclimated and then they go out and buy many fish. You are supposed to start with 2-3 fish to see how the tank does with them. If the tank does well with the fish then its fine to add more if the tank does not do well then you should make the nessacery ajustments.
Hope This Helps
Kyle