Hello Spader --
A few questions:
How long did you quarantine the new fish for? Normally, they should be quarantined for a few weeks. By the sounds of things, you moved the new fish from quarantine to the big tank within one day.
Are you keeping the mollies in brackish water? Whatever else you may have read, mollies are much more reliable kept in slightly salty water. Add 5 grammes of marine (not aquarium or tonic) salt per litre of water. Guppies, platies, and swordtails are fine with this amount of salt, as are an assortment of other freshwater fish. However, catfish, tetras, barbs, and gouramis (including bettas) generally don't like salty water. Hence, you should only keep mollies in a livebearer aquarium.
When mollies are kept in freshwater they are subject to a
variety of problems, all of which can be avoided by using salt. Even though you retailer will tell you they don't need salt, they do.
Are you keeping ratios of 2-3 females per male? Anything less leads to stress on the female fish, and stress leads to sickness. Never, ever buy livebearers in "pairs".
Fish should never be allowed to eat their dead companions. This is an excellent way for diseases to go from the dead fish to the living ones. If you, consider moving the betta to the 5 gallon tank, and leave the livebearers and the shrimp in the big tank. Add salt. Amano shrimps are fine in salty (brackish) water: they live in salty water in the wild, and will only breed in salty water. With luck, the salt will prevent any more fish getting ill, and you can certainly try keeping mollies again.
Cheers,
Neale
PS. What have you got against capital letters? You don't seem to like using them. Makes something difficult to read because the reader cannot see the breaks between sentences. Paragraphs are good, too, as you can probably tell by looking at my reply. The easier something is to read, the more replies you'll get.
