Sudden Balloon Molly Death?

[TryingMyBest]

So a balloon molly of mine (fairly new) began looking extremely round a few days ago. This happened to another molly of mine, who we thought was pregnant but ended up just developing her shape. This molly, however, became larger than any other molly in the tank. Yesterday I noticed she wasn’t eating and was staying near the bottom of the tank near the heater. Taking this as confirmation she was pregnant, and knowing if I moved her too close to labor she would abort the babies, I put her in a breeding box. The next morning I woke up and she was dead. I have a Topfin 20 gallon tank with all artificial plants, five other balloon mollies, and two babies (which are doing fine, got them from the pet store with my most recent molly because it was only her and the two babies in the tank). I have a pantyhoe over the filter because of the babies, could this be the problem?
TLDR: Did a pantyhoe over the filter cause the sudden death of my possibly pregnant balloon molly?

 

[Colin_T]

Hi and welcome to the forum

Pantyhose are unlikely to poison the water. Fish that are poisoned from chemicals usually gasp at the surface and all the fish in the tank die, not just one.

Fish that get fat suddenly (overnight or in a couple of days) usually have an internal problem like a bacterial or protozoan infection. The scales on the belly usually stick out sideways from the body, the fish might do a stringy white poop, they stop eating and either sit on the bottom or sit under the surface and breath heavily.

Balloon mollies also have problems with their digestive tract due to their body shape. Their bodies have been shortened and their internal organs have been squished up. This means they have trouble digesting food and are more likely to develop internal issues.

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How long has the tank been set up for?
Is the filter run continuously (24/7)?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do water changes?

Have you checked the water quality for ammonia, nitrite, nitrate, pH and general hardness (GH)?
If yes, what were the results in numbers?