Clamped fins are usually caused by poor water quality, a dirty tank and filter, and possibly external protozoan parasites, which often occur in dirty tanks or tanks with poor or blocked filtration.
Mollies regularly develop a disease called shimmying whereby they swim but don't really go anywhere. This can be caused by poor water quality and low pH & GH (mineral content in the water).
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If a fish ever gets sick or one dies from an unknown cause, do the following.
Test the water for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
Post pictures of the remaining fish and the whole aquarium so we can check them for diseases.
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Right now I would clean the glass and filter. Then do a big water change and gravel clean the substrate every day for a week.
