You can either let the tank sit 'fallow' (empty of fish) for about eight weeks or you can remove and boil the gravel, replace all the water and replace or boil all of the filter media and decorations.
Inside the tank as it is for you to see the water temperature.
It is used to regulate the temperature of the water
Placing a thermometer in your fish tank is important to monitor the water temperature. It helps ensure that the water is within the optimal range for your fish species, as fluctuations in temperature can stress or even harm the fish. Regularly checking the temperature can also help you detect any issues with your tank's heating or cooling system.
The temperature of the water depends on the type of life/fish you intend to keep in the aquarium.
To prevent shock which could kill your fish.
Yes it is important to have a thermometer in your fish tank. The temperature of the water should be between 75-80 degrees.
The instrument that is needed to measure the temperature of the water is called an "Aquarium Thermometer".
A heater can raise the temperature of a fish tank if needed
The water in the tank that you are transferring the angelfish to should be the same temperature and PH of the old water.If you are adding them to your tank from a fish store, then you should acclimate the fish first by floating the bags in your tank, and every so often add a bit of your tank water to the water in the bag to avoid temperature or PH shock.
Yes you can but I don't know what you hope to achieve by doing it. All you will do is reduce the amount of oxygen in the water and make it harder for the fish to breathe.
the name of the salt water fish is not given. more to the pointy is that gold fish are fresh water fish- therefore salt water fish other than anteroom's or catadromos fishes would eventualy die. These fish die because the salt water is cooler than claen water. Also, it would depend on what the temperature of the tank was before you changed it.