Bettas are opportunistic in many ways. His nest is there "just in case a lady drops in". If you are getting problems with water quality there is no reason why you can't change some.
Yes and I would recomend it.
the surface water would lower and lower, and eventually dry up.
Because of surface tension on the top of the water. Surface tension is when the water molecules stick together allowing a small, light object to "float" on the surface. If you genitally pushed on the pin after is settled on the water, you would break the surface tension and the pin would sink.
Due to the property of surface tension water surface would act as a stretched membrane. So water skaters, mosquitoes could easily walk on the surface. If oil is mixed with water then surface tension would fall and mosquito cannot breed. That is why we sprinkle oil in fine droplets on stagnant water
No it would not.
No it would not.
I think that would be surface water.
Fill it with water; it sinks. Fill it with air, it rises.
The salt water would move upward because salt water is less dense than non-saltwater.
No, it would be less due to compression by weight of the surface area water. Every so many feet (meters) down, another "atmosphere" of pressure is exerted. Ergo, water at the seabed would exert more pressure on the pier than the water at the surface.
Water has a property called "surface tension". This causes the surface molecules to be attracted to each other so as to form a 'skin' on the surface. This will allow an object that is heavier than water to appear to "float" on the surface, but NOT due to bouyancy alone. If you were to put a surfactant (soap) in the water it would eliminate the Surface Tension, and the floating arrowhead would then sink.
Water stick insects favor being on the surface of the water because this is where their food accumulates. Also, if they were under the water, they would not survive.