mollys tank, are they sick?

Question
Our molly are mixed with neons,tetras and pleckos.  We had a huge healthy counts of colorful molly (with several generationin the same tank)and recently added black mollys to the tank.  Since then we've lost several of our colorful mollies,also 1 neon and 1 tetra.  However, our black mollies do not have any spots, no swelling or no eyes trouble.  Recently the young adult black molly appear to be dying.  They are staying at the bottom of the tank and swim up in spirale,or are barely moving,notcoming up, or simply are lying low.  Could it be an equilibrium problem?  Our last tank had a mix of small aquarium rocks and large river rocks mixed toghether.  This tank has only large river rocks (approx. 1-2 in. long) build up to leave them room to swim and hide underneath. These rocks are covering about 50 % of the tnak's bottom, beside a few other thigs eg; a plant, yet there are several empty space at the bottom where you can see the bottom mirror/glass.  Right now they are staying in the corner of the tank where there is no rocks.  One is right in the middle of the rocks at the bottom.  It also has a live plant that came with the aquarium and the black mollies.  Could they be pregnant?  They don't appear to have any disease at all, and my husband works miracle usually with all animals, incl. fish, thus explaining so many generation of colorful mollies (now mostly gone).  Our mollies also don't apppear to be fighting and they are not missing any fins parts or tails ect...  Please advise.  Should we separate them?  If they had laid eggs, and are staying with them, Would we be able to see them ?  Please advise before they all die. Also could it be the fact that they see themselves at the bottom, via the glass mirror, and would appear perhaps to them as bottomless?

Answer
Hello David and Helene,
It seems as if the cause of the problem started when you added the black mollies, judging by your story. Most likely they carried in some sort of illness that is beginning to affect your entire tank. Do you have a quarantine tank? It would be good idea to remove any fish that look sick and keep an eye on them, in a separate tank. You might want to get a sort of cure-all, since the fish aren't showing any really specific signs yet. A good one to use is LifeGuard, it can cure several things at once and is still gentle enough to usually do no harm to fish that aren't actually sick. You might get two and put one in both the main tank and the sick tank. Don't forget to continue treating for 3-5 after the fish appear to have recovered, to prevent a recurrence.
In the future, definitely use a quarantine tank to let any problems new fish might have develop outside of your main tank, so they can be treated and the fish you already have will be safe.
Also, have you done any water testing, and if so how are your ammonia and nitrite/nitrate levels? And how large is your tank? With so many fish, I should hope you have at least a 30 gallon or larger. Be sure as well that you are not overfeeding, especially since you've added new fish because that alone can throw off your biolfilter. If you test your water and anything is higher than safe do a 30-45 percent water change immediately, and preferably use a tap water conditioner that has slime coat replacers as well to help your sick fish recover faster and any fish that appear healthy to stay that way. Lastly but not least, you might want to consider the possiblity that there simply isn't enough aeration in your tank. If you have no kind of bubblers or even a biowheel, your fish might simply be suffocating.
Best of luck to you!