Because the fish need the enough amount of water every day of their lives
evapouration
It will cool down water temperature when by melting down AND it will lift up the water level in the aquarium
Evaporation.
The fish are drinking it. Get rid of the fish
You need to get a water pump and filter for the aquarium at a pet store.
Because some of the water is evaporated into the air. Salt water is evaporated much faster than fresh water.
The ammonia level may still be high after a water change because the beneficial bacteria in the aquarium may not have had enough time to establish and break down the ammonia. It is also possible that the tank is overstocked or overfed, leading to excess ammonia production. Regular water testing and maintenance can help address high ammonia levels in the aquarium.
There should be a line indicating how far they go down in the water
You can do a water change while it is cycling, but that might slow down the process.
As far as the running of the tank and the efficiency of the filter is concerned, it doesn't matter if the water splashes down into the tank or flows quietly into it provided you are getting a maximum flow and the filter is cycled.
To prevent aquarium plants from floating in the water, you can anchor them down using weights or plant weights. These weights can be attached to the base of the plants to keep them in place at the bottom of the aquarium. Additionally, you can also trim the roots of the plants to help them stay grounded in the substrate.
the water level would sink. Anything that sinks into the water would cause the water level to sink