I have kept this white lyretail molly for a couple of months now, and my water conditions are perfect, according to a test. I discovered it stuck to the filter with a brown spot, a little behind the gills. My question is what is happening and how can I prevent it?
If the fish stuck to the filter, it means either it was already dead or it was extremely weak. Healthy fish normally would not be stuck to a filter. Just what readings do you get from your last test results? Ammonia? Nitrite? Nitrate? PH?
How often do you feed? How often and how much do you do partial water change?
Has the Molly been giving birth lately?
I have also found that livebearer seem to shorten their life span quite a bit when they give birth frequently. Several of my Platy (also livebearer) didn't live long when they were pregnant constantly. While a few other female Platy in the tanks without any male has lived for much longer time.
My last test results, the pet store told me they were perfect (they give free water tests). I have two mollies in it (one slightly larger black lyretail molly). I have not seen my molly give birth yet. I do partial water changes once every two weeks (i change out about 1/2 to 1/3 of the tank). I feed it two mashed up pellets for pond fish. The pellets are about the size of corn. I noticed a week ago that my molly had a dark spot (black)behind the gills. I thought it was pregnant, because its belly was very large. the black spot disappeared, but came back with the brown spot. My black molly appears to be fine now. I have noticed that the white molly used to chase the black one and nip its tail when feeding. The black molly gets fed, and its bigger too. However, there came no noticeable damage, so I ignored it. Although you said it could be extremely weak, I doubt it would heal back anyways, so I took it out of the tank just in case it carried any disease or parasite. I have the tank filtered day and night, with a heater that keeps the temperature at about 80 degrees Fahrenheit. I dechlorinate my new water with this liquid that my pet store recommended. I recently cleaned my tank (7 days ago). Also, I fast my fish 1 time a week.
Additional Info:
My molly tank is about 9 gallons with gravel on the bottom. It has a pretty strong filter with two filter media slots. I keep both filled. 80-82 degrees.
Water testing at pet stores are not the best choice. Many of them always say it is perfect when it is not. You need to get the actual readings instead of the word "perfect" from them. Better yet, get your own test kit. API Freshwater Master kit is cheap, and you can do 100~150 tests with it at home, no need to go out and get your water tested. Totally worth the investment.
Weekly partial water change is recommended instead of once every two weeks. You've also been feeding too much. Molly can't eat two corn sized pellets per day. Those two pellets are more than enough for a dozen Mollies.
I am also interested in how you cleaned your tank 7 days ago.
Often when people are not careful when they clean the tank, they might crash the nitrogen cycle or partially crash it into a mini-cycle. When I clean my fish tanks, I do not go beyond clean the side of tanks with algae scraper or 100% cotton clothes, plus vacuum the substrate. Did you clean the filter? If so, how did you do it? That part had to be done extremely careful under old tank water or (treated) tap water, or the beneficial bacteria could be killed by chlorine. The filter media also must stay wet the whole time.
Better yet, get your own test kit.