My platies seem to be getting sicker by the minute...HELP???

Question
I have two Mickey Mouse Platies (one male and one female) in  five gallon tank, they have been doing fine since we got them three weeks ago, but since introducing a new M.M. platy to the tank (female, which died less than 10 hours later) my two other platies have been laying on the gravel or on the plante leaves and not swimming around a whole lot, I have checked all levels (all on normal ranges) and did an emergency water change just encase, but they are still not doing any better...please help. These two fish are the longest kept fish in this tank since we got it in May 11'. We have bought like 20 fish (like three or four at a time)and all except these have died. Don't know what I'm doing wrong???

Answer
Hello Becca!

I understand that you are concerned about your beloved pets but here is what you can do. Clean the filter, air stone (or buy a new one)and buy the API Stress Coat & Stress Zyme. They lower stress levels up to 40% (scientifically proven) and makes your tank cleaner @ the same time! Stress Coat also has the healing power of AloeVera which will be very helpful in this case. What you have done so far really helps! Since you have bought like 20 fish over time, new bacteria may have been also transported from the petshop to your tank. Add a little salt for disinfection. Watch for results.

If anything gets worse, contact me ASAP.
Hope this helps! :D
~Sam