Most heaters are designed to be used in tanks that are a minimum of 2 gallons. 1.5 liters is beyond unreasonably tiny, even for a temporary tank. That is less than half a gallon and you must do 100% water changes daily with a dechlorinating product such as Seachem Prime in order to keep the fish's waste under control.
Fish constantly excrete ammonia from their gills, kind of like their form of urine. This ammonia is extremely toxic, and having any detectable amount of it in the water is unacceptable. The only way to maintain an undetectable amount is through very frequent 100% changes. In nature, this ammonia would be converted into less toxic compounds by beneficial bacteria, consumed by plants, and diluted over much larger quantities of water. The container which you are speaking of would turn into a death soup of ammonia in very little time.
I highly recommend getting a larger temporary container. You can use rubbermaid/sterilite plastic storage bins like you would find at WalMart or Target--these bins are very light, durable, and they're safe to use with your heater. A 2-4 gallon bin is only about $3, you can't beat that. Keeping your fish in such a small container is very impractical, and you won't be able to use your heater. There are many cheaper and more humane alternatives that won't have you changing the water so much. If you can, you should return the tiny deathtrap to the pet store--the pet industry and the retail store should not be allowed to profit from the sale of these torture devices.