Why does my molly fish tank smell after babies? How to prevent it?

Hard to say from your description whether she is pregnant or ill. You can tell male from female Mollies by looking in the area of the anus. A male Molly has a so-called gonopodium. The gestation period of a molly fish is about 60 days. Female mollies can store sperm for months and may fertilize eggs as often as every 30 days, even without a male in the tank. Mollies may release 10 to 60 live babies at a time. Here is an illustration of sexing Mollies (and other live bearers)(credit: Mwahahahaa) http://photobucket.com/gallery/user/Mwahahahaa/media/bWVkaWFJZDoyMDk0MTEzNw==/?ref=

Black spots or discoloration can be caused by elevated ammonia levels. Test water chemistry using a home test kit or submit a water sample for testing. Most pet or fish stores will do this for free or for a nominal fee. Ammonia and nitrites should be 0.25 ppm (parts per million) or less, nitrates can be 40ppm or less. Partial water chnages daily should be done to decrease ammonia levels. Use only aged or conditioned water that is temperature matched to prevent further stress or shock. Elevated ammonia levels can be caused by overfeeding, new tank syndrome or overstocking.

The natural antibiotic properties of Melafix work best with minor or external infections. Your Molly may be quite ill and may need more aggressive treatment. If you have other fish in the tank, especially Bettas or Gouramis, you cannot use Melafix as this could harm them. Separate the molly to a hospital tank and treat there. You will need to remove the carbon filter insert during treatment or it will filter out the Melafix. The symptoms listed may indicate bacterial infection, parasites, constipation or obstruction. If Negrito has a pinecone appearance, he may have a serious infection (dropsy) that is causing kidney failure. This would in turn cause the swollen abdomen and lack of appetite. Check the water chemistry, temperature and filtration. Ammonia and nitrites should be absent and nitrates can be present at 40ppm (parts per million) or less. Molies do best in water between 75F and 80F. Perform a partial water change if necessary to stabilize chemistry and maintain water quality. Use only aged or conditioned water that is temperature matched to prevent further stress or shock. Clean water is the best way to manage any infection. Plain aquarium salt (NOT table salt) can be used at 1 teaspoon per gallon of water but it is not recommended if your molly has dropsy. In this case, Epsom salt can be used to decrease swelling. Allow a 15-30 minute swim in a bath prepared with 1 tablespoon of Epsom salt per gallon of water. Do this in a separate container. It could help with constipation as well. If the Melafix does not help, use a product such as Kanaplex or Maracyn. These are broad spectrum antibiotics that you can find in the aquarium section of the local pet or fish store. Follow label directions carefully and remove the carbon filter insert during treatment if indicated. Hopefully, any infection will be resolved with treatment. If it is dropsy, unfortunately, this condition has a high mortality rate.

Mollies do come in gold or orange colorations, but it is difficult to determine from that angle. If you can repost a broadside picture in a new question, that may make identification easier. If it is an egg laying species of fish, the third image could be an egg but I have reservations saying definitively without something for scale. It looks too large to be an egg. As an aside, if you leave the Molly fry in with adults, they will likely be eaten. You can separate the out, remove the adults, use a breeder screen or provide plenty of cover for them.

Depending on the size of your tank, 1 or 2 mollies should be fine. The one issue you may have is getting food to your ACFs. You may need to use a turkey baster or syringe to get the food to them before the mollies eat it.

I agree and do not recommend to move the female. If your tank is big enough, you can usually purchase a breeder net and keep the babies in there. Otherwise I recommend to purchase many fake plants...etc to provide hiding so that some of the babies make it. Best of luck.

Check the water chemistry and quality. You can use a home test kit or submit a water sample for testing. The local Petco may do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with nitrates at 20ppm or less. Elevated ammonia or nitrite levels can be toxic and could drive the fish to the surface. Water that is too warm may also do the same since it holds less dissolved oxygen. This can happen if water temp is above 82 F. Perform a partial water change, if necessary, to stabilize chemistry and/or to improve water quality. Use only conditioned water that is temperature matched to prevent stress or shock. To treat the swim bladder issue, fast her for 3 days, then feed bits of cooked and shelled green peas until swimming and defecation are normal. Constipation is a common cause of swim bladder disease. You can also consider a broad spectrum dewormer since parasites may also be the cause. Find one in the aquarium section of the local Petco. Follow label directions carefully and remove the carbon filter insert during treatment if indicated.