Looks like it's going to be a long time before we get any results on Fred's mercury/selenium content in his tilapia, which uses the Big Island black cinders for his bio-filter.  Everything up to this point would deem it safe, but for the official results, here's my last response from Clyde:

Re: Fw: Mercury in Lava Rocks

FROM:   Clyde Tamaru

TO:     Larry Yonashiro

Thursday, November 3, 2011 6:36 AM
 
Hi Larry and I pick up the last two wild kahala that are to be used to test for mercury and then still need to get some wild chinese catfish.  The DOH folks want me to bring all the samples in at once so not going to have the results in soon.  Not a cost issue but more of a time one at the moment.  Need to catch some wild chinese catfish for me to help speed things up...  Sorry for the incovenience ...

On Thu, Nov 3, 2011 at 5:53 AM, Larry Yonashiro <lyonashiro@yahoo.com> wrote:

Hi Clyde,
I hope you are well.  Hate to be a pain in the butt, but any progress on this?  I need to report back to some people who are actually waiting on your results before they build their bio-filters.
If it's a cost issue, maybe some of can share in the cost for this.
Mahalo,
Larry

I'm all about using media beds as filtration for inline raft systems. IMO black lava is the only media to use because of the cost, availability, & surface area. I was interested to hear about the mercury/selenium issue, & will follow this post.

Thank you to UH, & our Ag Extension Office !

This is a good thread with excellent points made by experienced and new growers.  Best to use grow media beds followed in series by a flow bed.  Each has its strengths as pointed out below.