silver lyretail molly sick

Question
I have a 20 gallon tank, with some guppies(4), platies(3)a pleco and 2 silver lyretail Mollies.  the female just had babies about 4 days ago, and the male just died 2 days ago. all the other fish have been in the tank now for a long time (year).  the Mollies were added about 2 weeks ago.



the symptom are that they hang at the top vertically, it almost seems as if their tails were paralyzed--like they couldn't move it.  the flutter only their front fins.  they don't seem to have the pine cone look, their appetite has become poor, and initially their "mobility" seems to get worse after eating but it's hard to tell since they have little interest in food.  The female is acting this way now too.  i think she is doomed but I'd really like to save her.  I add 3 TBSP salt to my water changes (about3/week) i stepped up the water changes when he started showing signs and added 2 TSP of pima fix--he got to the point that he would "stick" to the filter tube.  I cant tell if its ich since it's hard to see white spots on a white fish..
it would appear that one of my female guppies has the same problem.  she sinks down to the bottome and her tail seems to flop downward rather than straight across.

i did another water change last night(approx 50%), added 2 TBSP of salt


Help?

Answer
Have your complete water parameters (ammonia, nitrites and nitrates) tested at your LFS and get back to me with the results.  It really sounds like old tank syndrome.  Your tank was able to handle the bio load until you add the Mollies, now it can't keep up.