Steve, the threads are external - have'nt tried mine on any skimmers yet, but been thinking of putting it on a AquaC style skimmer - great pump. 
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Mike
150g reef, 55g sump, T5's, Vertech 200A, Profilux III - German made is highly over rated, should just say Gerpan made.
Reefkeeper - individual obsessed with placing disturbing amounts of electricity and seawater in close proximity for the purpose of maintaining live coral reef organisms.
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