Blackmoors swim to the bottom when filter is on?

First of all never turn the filter off, this will cause the beneficial bacteria that live inside the filter media (the sponge or ceramic rings inside the filter) to die off, resulting in poor water quality which could be having a serious effect on the health of the fish. Most internal filters have an adjustatable output (usually a small knob or lever on the filter) but if you can't adjust it you can deflect the flow by putting some plants in front of the outlet.