FLUVAL U MINI. Fluval U1 Underwater Filter provides efficient filtration to create superior aquarium water quality. It is also ideal for tanks with low water levels. AQUARIUM CAPACITY | up to 45 L. In addition, the filter can be installed vertically against the aquarium wall to create currents or customized flow patterns; or it can be placed horizontally in shallow water tanks; or it can be used to create decorative features such as waterfalls.