Wierd things happening to my mollies

Question
Hi Craig.  I have a 55 gal tank, with several varieties of fish including, of course, mollies.  First off, I clean the tank/water change once each and every week, religiously.  Water parameters are in the norm, Temp. 78F, feedings once to twice a day--a variety of flakes and live foods is given.  A few months ago I noticed my sailfin had raised scales and bloating which I diagnosed as dropsy.  I treated the whole tank-removing the carbon filter-for 1 week with Melafix.  Things were back to normal after that.  Then 3 weeks ago my balloon molly started sitting on the gravel and died the next day.  Two days ago the sailfin went belly up (but otherwise looked normal).  My pale (kinda pink hued) molly female over the last 6 months had gradually began to change to a brown color on most of her abdomen--but she acts normal. . .then today I noticed pimple-like things on the bottom of her abdomen.  I have several plants and cannot put salt in my tank.  Do you have any idea what could be going on?  Is there some strange disease affecting my mollies, or are they just getting old (I've had them for about 2 years)?  Thank you for any help you can give,
Margo

Answer
When you clean the aquarium how much water do you remove?  The aquarium temp sounds good as does the feeding schedule.  It sound like you might have a parasite in your aquarium that is attacking fish with a low immune system, I know you said you have plants in your aquarium but Mollies doe require some salt in the aquarium to be in peak condition, if you put a salt mixture in the amount of 1tbs per 5 gallons in your aquarium your plants will be fine and it will boost your Mollies immune system, it will also help perk up the other fish in your tank as well.  I would also increase the temperature to about 82F for a week, between the salt and temp increase it should kill off any parasites.