Mollys sitting on bottom

I have a pair of mollies that I have had for 2 months in a 20 gallon aquarium. They were very happy during this time, and even had babies. Recently, the male started to sit on the bottom so I transfered him into a 5 gallon hospital tank. This tank was house to a pair of zebras that are now in a totally different (third) tank. Now the female molly has been acting the same way, so I transferred her to the hospital tank also. The readings on both tanks are 0 ammonia, 0 nitrites, and low levels of nitrate. Both mollies spend all day sitting on the bottom of the hospital tank, where as they used to swim around actively all day long. They will swim to the top and eat when I give them food, but then promptly return to sitting on the bottom when they are done. I have added a bit of salt to the hospital tank after reading that mollies sometimes prefer salt in their water, but it does not seem to have changed their behavior after about 1 week. They do not appear to be suffering from any diseases and still look the same as when I bought them. I'm running out of ideas and was hoping that someone might have more suggestions.

Thanks for your time and responses.

Thanks for your replies.

They are still hanging out at the bottom of the tank, though they do swim to the surface when they see me looking at them. They will also eat when given food. They do seem to show signs of heavy breathing, but they're fins are very actively moving to stay in one spot.

The salt that I added was table salt, about 1/2 tablespoon, into the 5 gallon hospital. I change about 20% (1 gallon) of water weekly and use water conditioners to remove clorine and add electrolytes to the water (NovAqua I think and the other one that came with it). I also make sure the temp of the added water is around the same as the tank.

Two weeks ago I was mixing an antibiotic in with frozen brine shrimp and feeding, but stopped when they did not seem to improve any. I treated them with it for 1 week. I can't remember the exact name of the medicine, but it starts with an "M" and is a white powder. It was expensive too, like $12 at the LFS. I am now wondering if this antibiotic was more for infections than for internal parasites. At any rate, I have stopped treating them with it.

Does anyone have any recomendations for internal parasites? I am beginging to wonder if they have the begining stages of dropsy, as there was a female dwarf gourami in the original 20 gallon that died and was "pineconed", but this was like 2-3 months ago. On a side note, none of the other fish in the 20 gallon seem to be acting any different, thankfully. I'm just glad I was able to seperate them out.