a big one!
For a GloFish tank, the best type of heater to use is a submersible aquarium heater. This type of heater is designed to maintain a consistent water temperature, which is important for the health and well-being of GloFish. It is recommended to choose a heater that is specifically designed for the size of your tank and to regularly monitor and adjust the temperature as needed to ensure the optimal environment for the fish.
A heater can raise the temperature of a fish tank if needed
One can purchase a fish tank heater at various pet supplies shops. PetSmart, Petco, Marine Land, and Argos all have fish tank heaters for sale on their websites.
Circulation
If you have a heater, turn it off.
FIlter, heater, tank
Usually there is a heater that warms the tank on purpose.
4 feet
Most fish are around 78 degrees F.
12 fishes
The only thing that you can put in a tank that small is a betta fish or african dwarf frog. Tropical fish do better in a tank that is ten gallons or more. If the tank has a filter and heater you could put one guppy into it but that would be all and I doubt it has a filter and a heater.
you can put in sea snails and more brightly fish that you cant get with a heater in your tank