food colouring would give the best visual indacation of flow but might be a pain in a reef tank as you would have tyo get it all out befor the second test 
Steve
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 *everything said above is just my opinion, and may or may not reflect the views of this BBS, its Operators, and its Members. If cornered on any “opinion” I post I will totally deny having ever said this in a Court of Law…Unless I am the right one*
Some strive to be perfect.... I just strive.
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