Black Molly - Badman's Tropical Fish

Tropical Fish for Freshwater Aquariums: Black Sailfin Molly Livebearer
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I see some potential problems with your fish aquarium and I am going to address each one separately. To answer the first part of your question about the ill black molly: I think chances are the blue growth or discoloration you are seeing may be a fungal infection. Extreme stress from unstable water parameters can cause fungal infections and from the symptoms you described it is my guess that bacterial fungus has made your tropical fish sick. Treating fungal infections can be very difficult and the most effective way to handle these is with an antibiotic.
The Black Lyretail Molly (Poecilia latipinna) is a tropical community fish
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The Molly is a tropical fish that prefers a little salt in their water. A teaspoon of aquarium salt per 5 gallons of water will go a long way in helping them. You may also see them in saltwater tanks from time to time. There is a local reef store here that keeps black molly fish in a regular saltwater tank with similar sized species. The molly is a very attractive tropical fish that comes in many different colors such as orange, green and black. Some of the more popular varieties include the sailfin, balloon and the dalmation. Black Molly - Swordtail Half Breed - tropical fish tank update - YouTube
Photo provided by FlickrBlack Molly problems - Badman's Tropical Fish
Photo provided by FlickrMolly (Black Molly) -for- Warm Water Beginner Tank (Probably Tropical Fish?)
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Black Molly - Swordtail Half Breed - tropical fish tank. I gather they call that a cross-breed Molly/Swordtail. Here is an update on OBI, as we named him (Orange Black kenobI). This is a mix breed. Half Black Molly and half Tropical Swordtail fish. Here was original video of the Black Molly-Sowrdtail fry:

In the first video posted on Christmas 2012, some commented that this was biologically not possible. Mollies en Swordtails cannot breed with each other. Then again, others said Swordtails can breed with Black Mollies. I think that this is, proof in the pudding - proof in the aquarium - a Black Molly and a Swordtail can breed and produce fry or baby fish!

"can mollies and platys or swordtails breed?" See forum post:

Half Breed in the living of a Black Molly - Swordtail cross-breed or mix on video. Black Molly - Swordtail fry!Mollies generally get along with most other tropical fish, and aren't prone to territorial aggression. There is the propensity for new molly owners to overfeed them, however, and they may steal food from other members of the tank. Feed black mollies only what they can eat in three minutes. If they are breeding, you should include live or frozen foods as a part of their diet. Bloodworms, brine shrimp and blackworms are all excellent choices.Black mollies are hardy little tropical fish that can tolerate water ranging from fresh to brackish, but thrive in freshwater best. The black molly is an excellent starter fish for those new to aquatic hobbies, especially if single sexes are kept. If you own both male and female black mollies, expect these prolific breeders to populate their tank with molly fry.