I have a 25 gallon tank last week I had bought a new heater with thermostat. It is 150 watts but the water seems not to be heating up. My Blue Line Shark has Ich so I raised the temperature to 32 degrees Celsius immediately. Can that also cause this problem after putting it in the tank?
150 watts in 25 gallons is plenty in normal heated room. Is the heater on? Is it set correctly? There is usually some light indicating that the heater is actually heating (if not, test it carefully with your finger - is it hot?). If it is going on and off and the temperature in the tank does not rise, it may be caused by insufficient water movement around the heater - point the filter outlet to the heater to increase the flow around the heater.
On a side note, 25 gallons is way too small for sharks. If the tank is used only for the treatment, OK; if it is their normal home, it is a problem. It may stress the fish and cause this, and many other health problems in the long run.
what is the ambient temperature in the room? try setting the heater a few degrees higher and see if that helps. In case the ambient temperature is low, the heater might not be able to help much. A higher wattage heater may help. Also bear in mind that a heater will cause loads of tank water to evaporate, which may need frequent replenishment.
I'm too lazy to look up the Celsius Fahrenheit difference but if your fish have ich I'd recommend bringing your tank temp up to 82F degrees and get API Super Ich Cure along with some aquarium salt. I've saved a lot of fish using that combo since I used to work at a fish store.
You can read up on how aquarium heaters work against room temp here Aquarium Heater Size Guide