By reading the directions that come with each heater as each one is slightly different
Gas water heater has an orifice for main burner and for pilot light, if not electronic ignition.
Yes
The water continues to run into and out of the water heater. There is just no flame to heat it up.
If the pilot light is on, the heater should come on by itself. If the pilot light is off then it has to be lit. If it will not stay on, the thermocouple is bad.
Have you turned the control from Pilot to On? Have you increased the temperature setting?
No, it is not dangerous. (Unless you are working with a water heater that is 20+ years old.) In any modern hot water heater, if the pilot light goes out a fail-safe will go off - preventing gas from leaking out. Also, there is a sensor so if there is not enough air-flow or oxygen with the water heater, it will shut off the gas.
If the tub is not getting hot water the issue is probably with the hot water heater. The hot water heater may have tripped a breaker or the pilot light went out.
Same as any hot water heater. Turn the gas valve to off.
If you have a gas water heater and your pilot light will not light, the most common problem is a faulty thermcouple. A licensed plummer can easily and cheaply fix this or you can find information to do it yourself online.
Yes, as the day goes by and the heater and as well as the light are turned on they make the water "disappear".
You will need to know the current water temperature in your tank by using a thermometer, so you know if you need to increase or decrease the water temperature for your fish. Take your heater, place it in the tank according to the instructions, DO NOT plug the heater in for at least 20 minutes. After plugging the heater in, slowly turn the thermostat up until the heater light comes on and turn it back until the light goes out, if the light is on when you plug the heater in, slowly turn the thermostat down until you find the point where the light goes out. You have now just adjusted your heater thermostat to the current water temperature, turn it up or down as needed, but only by a small bit and allow 24 hours for the water to heat up or cool down, before adjusting the heater again.
THERMOCOUPLE or defective gas valve