Do they look like the ones pictured here?
http://www.wetwebmedia.com/amphipodfaqs.htm I’m guessing you are seeing Scuds or Gammarus shrimp (Gammaridean amphipod). They are beneficial detritivores. You want to keep them in your system. They are natural foods for you fish. You may want to do some reading about refugiums, you will likely learn a lot. More here and related links in blue: http://www.wetwebmedia.com/ca/volume_2/cav2i1/Pods/pods.htm http://www.wetwebmedia.com/refugium.