The humidity should go from 55% during incubation up to 75% for hatching. It's no really the amount of water in the tank, but the amount of surface open to the air. You have a towel in the tank, which will reduce the surface and therefore keep the humidity down. It is better to have some sort of screen instead of the towel. This can be a shelf liner for a window screen. You can also have strips of cotton cloth hanging down the sides of the tank reaching into the water. The strips will wick up the water and expose more air to the moisture. If you do that then you can keep the towel under the eggs. Also make sure the ducks cannot get into the water below. Hatching ducks love to drown themselves in water. Keep the duckling in the tank for another 12 hours after hatching. After then you need a brooder. A large tub with a regular light bulb should work. You do not want to overheat the ducklings. I start off with a 100 watt bulb 2 feet above the surface and reduce the wattage each week. They do not need at hot red heat lamp. I put feed in a small 4 ounce canning glass mixed with water for the first two days. Same for water. You do not want them to play in it. Just for drinking. Next to that a small glass of sand when they are three days old.
Katharina