All my ferneries are made up of 3 layers before the soil layer to ensure drainage is 'mimicked' similar to an open terrarium. The evaporation level for the husk ferneries is such that they are more prone to drying out than becoming stagnant. The stump ferneries can go for longer in between watering due to their depth although still benefit from misting.
No, it's not dying but it is getting eaten - by caterpillars. Even though you may have your fernery inside there are still moths than can lay tiny eggs. You would be surprised the amount of damage they can do in a very short time.
The bioinsecticide included in your purchase is useful to slow down the caterpillars eventually but it will not kill them immediately like a heavy duty insecticide. For this reason a brief brush of your hand through the ferns is usually enough to dislodge any caterpillars - they drop and you can spot them as bright green (very similar in colour to the ferns which is why they're tricky to spot). Then just remove them.
If you find a large brown one they can do as much damage as a lawnmower - remove immediately!
Remember, your ferns will grow back, just trim down in order to encourage new fiddleheads.
I recommend a good drink of water (especially for the husk ferneries) and a misting then leaving your fernery outside in full shade and preferably off the ground (so it doesn't get damaged if it rains). You can try leaving under a table if you don't have much space.
Alternatively you can call me to see if my holiday care is operating at $10 a week (not available all year).
Stop watering! You have overwatered your fernery and it's screaming out for a break. Most ferns are actually epiphytic meaning they're happy living off debris on tree trunks and rocks. Your fernery is not a pond. Let it dry out before watering - it should be at least a week in between watering and can afford to go 2 weeks after a good watering if not located in front of fans or air conditioning. Feel free to mist as often as you like if you prefer to keep hands on with them.