Possible Reasons:
* Low Oxygen Levels: Mollies need well-oxygenated water. If the water isn't getting enough oxygen, they might hang out at the bottom where it's slightly more oxygen-rich.
* Poor Water Quality: Ammonia, nitrite, and nitrates are all toxic to fish. If your water parameters are off, your mollies might be experiencing stress and lethargy, leading them to stay at the bottom.
* Disease or Parasites: Internal parasites or bacterial infections can weaken fish and make them sluggish, causing them to stay at the bottom.
* Temperature Issues: Mollies prefer warmer water (72-82°F). If the water is too cold, they might become lethargic and stay at the bottom.
* Stress: Overcrowding, loud noises, or sudden changes in their environment can cause stress, which can lead to lethargy.
* Old Age: Older mollies might become less active and spend more time at the bottom.
* Constipation: Constipation can be a problem for mollies, and it can cause them to stay at the bottom.
What to do:
1. Test your water: Get a water testing kit and check the levels of ammonia, nitrite, nitrates, pH, and temperature.
2. Check for signs of disease: Observe your mollies closely for signs of disease like fin rot, white spots, or bloating.
3. Perform a partial water change: Replace a portion of the water in the tank with fresh, dechlorinated water.
4. Increase oxygenation: Add an air stone or increase the flow of your filter.
5. Check your filter: Make sure your filter is working properly and is the right size for your tank.
6. Adjust the temperature: Make sure the water temperature is in the optimal range for your mollies.
7. Reduce stress: Avoid loud noises, sudden changes in the environment, and overcrowding.
8. Consider a diet change: If you suspect constipation, try feeding your mollies high-fiber foods like blanched spinach or zucchini.
If your mollies are still staying at the bottom of the tank, it's best to consult with a veterinarian or an experienced fish keeper to get a more specific diagnosis and treatment plan.