25l is a bit over 6.5 US gallons . I have an Eheim 2211 running throttled down some on a 30l and it's running fine for the last 9 months or so , though I'm probably going to put a small HOB on the tank when I move it shortly. Consider : Eheim 2211 flows around 300l/hr , or a bit over 10X water change/hour . The next one up (2213) is 440l/hr , one up from there (2215) is 620l/hr . Do you REALLY need that kind of flow in what's really just a tiny tank?