Sounds like "the shimmies", a nervous disease that's quite common in mollies when they're kept in freshwater. Mollies *really* do need marine salt mix added to the water, however much some of us wish it weren't so. Because mollies sold in shops aren't "a species" but the product of hybridisation of half a dozen species (at least) you cannot predict by looking which mollies will do well without salt in the water and which will not.
So, the safest option is to add salt to the water. Don't use table salt or tonic salt. Only use marine salt mix. Why? Marine salt mix isn't just sodium chloride, but other mineral salts too, such as calcium carbonate. Taken together these raise the pH and hardness, and it is probably this, as much as the salinity itself, that seems to help mollies. You want to add about 5-6 g salt per litre (which is about 15% seawater salinity, or SG 1.003).
While livebearers such as guppies tolerate salt, most standard tropicals (barbs, neons, etc.) do not.
Usually, the shimmies is revealed by the molly clamping its fins to its body, rocking, and generally not being able to swim properly.
There are other possibilities, such as swim bladder problems. Constipation can cause swimming problems. How much green stuff are you feeding your molly? At the very least you should be using vegetarian flake food (a.k.a. livebearer food) and *not* flake food for carnivores (standard flake food). You should also be providing some fresh greens or algae. Blanched lettuce, thin slices of cucumber or courgette, and even bits of sushi nori work well.
Cheers,
Neale
I've got a problem. One of my mollies, which must be about 4 months old now, has suddenly curved its body and can only swim in a spinning motion. He can go up and down the tank, but can only swim in a circle to the left. Do you think he's diseased? He appears to be alright, as in not bloated or have any other marks on him.
