The problem could be the water and other elements in the tank. You need to start charging water regularly, and you can begin by performing a 100% water change.
Ammonia build-up has been studied to cause all sorts of ailments in fish. You need to ensure that the amount of ammonia, nitrites, and nitrates is below 40 ppm. To lower these compounds, you can do the following.
- Change water regularly.
- Scoop out unnecessary organic matter like food remains, rotting plants and dead fish.
- Vacuum/clean the gravel.
- Unclog the filter.
- Cut down on feeding.
- Avoid overstocking.
- Introduce healthy bacteria.
- Lower pH.
- Increase aeration.
- Use ammonia neutralizing/removing pellets/drops.
When performing a 100% water change, let the tank dry in the sun and soak its accessories in hot water. If you have plants, get them outside to vaporize any water on them. When adding fresh water, include some aquarium salt or conditioner, and ensure the water temperature remains between 26-27 degrees C.
After changing the water, cleaning the tank and obtaining the right water parameters, you can continue with the fungal medication.