I just bought a Top Fin Submersible Aquarium Heater, its the 25w for my 5 gal tank. Its in the tank, plugged in but the instructions don't say how to work it. It has a nob at the top and when we turn it all the way one way a light comes on and vice versa. The light goes off after a while (the instructions say the light is suppose to go off when at your chosen temp) but the water isn't near where it would be if it was at the highest temp (92F). Obviously I don't want it that high, but I would like to be able to get the water warmer for Tayln. Does anyone know how to work this heater?