For me, it seems helpful to remember the following regarding VFTs--
The sunnier and warmer it is, and the more the plant is growing, the more moist it can be.
The darker and cooler it is, and the less the plant is growing, the less moist it should be.
Problems such as the fungal problem you mentioned can happen when the plant is too soggy for too long, and is in chilly or cold temperatures.
What can make the situation worse is when the plant is not growing much nor very actively (like during dormancy), or when the plant grows too fast and becomes weak (like when it has too much water and not enough sun).
During dormancy or when it is darker and cool, try to keep the medium just moist, even on the dry side of moist for VFTs. Give it more water only when there is more light and warmer temperatures.
These are just my own personal guidelines. They may or may not be helpful to you. Best wishes and good luck--