You´ve raised another problem with the limited space.
Filters produce a current, and if you have a small water volume, the current will shake everything up. Since Cynops orientalis is a species that inhabits still masses of water, this is not good at all. In fact too much of a current will cause enough stress that the newt will abandon the water and refuse to use it.
I always recommend to avoid using filters with this species, because they really do like the water to be totally still (a very weak current is not a problem provided that the water volume is big enough). A big enough, well planted, cycled tank does not need a filter as the surface of all the plants will provide enough space for the bacteria to grow on to recreate the effect of having a filter, but without the current. The plants themselves contribute by taking ammonia and nitrites from the water.
If i insist so much in the fact that you need the whole 37l for the newt is because if you want to provide excelent care for him, you really do need all the volume. It´s possible to keep them in smaller volumes but that will always be sub-optimal and will raise problems. Your duty as a keeper is to provide the best care you possibly can, not to see if it can survive in less (which it can...but it won´t thrive).