NEUROTIC Mollie

Question
Hi,
I have a 20 gallon aquarium, with live plants, gravel and I bought 3 Mollies yesterday.
My tank has been set for 3 weeks. On the 2nd week I added my plants and now on the end of the 3rd week I added my fish.
I had no ammonia and no nitrite in the water, even after I added my plants.
Before having the plants my PH was 6.8 then after adding them it rose to 7.4.
After adding the fish I have now PH of 6.2, Phosphate of 2.0 and Nitrate of 5, still 0 Ammonia and 0.2 Nitrite

Yesterday, when I arrived home from the fish store, I start reading some articles about mollies and added to my tank some marine salt. It seems like it cheer them up almost instantly!

They swim up and down the tank, really playful ( this is normal right?)

So my Black Mollie (which I think it's a girl but I'm not sure) - she is really quiet, and stay separated from the others, although she seems to be very healthy.
The Dalmatian Mollie (which is a boy), developed a round, brownish spot on each side close to his guils, but besides this he seems to be a very active fish.
Then the problem is the Red Mollie (it's a girl) she is nervous, almost neurotic, it seems like she is unconfortable with something and I don't know what is it. The other 2 look calm, they swim around the aquarium, they seem to enjoy the live plants, but the red mollie she swims fast all over the tank, then she stops and start with those neurotic BACK AND FORTH MOVEMENTS, SWIMMING ON THE SAME PLACE AND BECOMES AGRESSIVE TOWARD THE OTHERS, it's not nice to look at her...she seems to be suffering and the others seem to be just fine...
PLEASE HELP,
I'VE TRIED TO PREPARE MYSELF THE MOST I COULD, BEFORE BUYING THEM, BUT I DON'T KNOW WHAT IS HAPPENING AND I DO WANT TO DO IT RIGHT.
Thanks for your time,
Melissa  

Answer
The red molly may have a little bacterial infection. The rocking back and forth is a symptom.  I would treat them with general cure to be on the safe side.  You can treat them with triple sulfate instead of general cure too... please keep me posted... everything else sounds good.  dave