By Bob Goemans
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Bob Goemans corresponds with Suzanne Robertson

Suzanne Robertson writes...

Dear Bob,

I am so fortunate to have found your site and all the wonderful insight you impart. Unfortunately I may have been given incorrect advice on plenum construction and substrate grain size. I built my plenum for my 120 gallon FOWLR tank according to directions received from an aquarium shop that advertising on the Internet. He recommended cutting a 1" hole in the plenum to insert a riser to run a powerhead while the tank cycles until ammonia and nitrite read zero and then pull it out. Although, he did say I could keep it running for a FOWLR tank. I like the design of the plenum as the risers underneath allow for a serpentine water effect with the risers cut shorter and longer through the middle, which mimics a more true "plenum."

The other cause for concern is the grain size. I asked him if using a mixture of Arag-Alive Fiji Pink and Indo Pacific Black would be ok and after coming across your articles it looks like I may need to take the system apart? Tank has been up and running with 120 lbs of fully cured live rock, substrate, and janitor crew for a week. Should I take it down and redo with correct grain size or not have the plenum at all since it is in the display and not in a separate tank? I have a 1" layer of sand over the plenum and 3" over the second screen. What would be your recommendation? Thank you for your time and consideration.


Suzanne Robertson

Bob replies...

Hi Suzanne,

Thanks for your email and nice words about my website. Seems like some still do not understand the difference between the UGF and plenum method processes. That's quite sad, as plenum 'facts' have been clearly and thoroughly written about for at least the past 'decade!' And as to the information given, it's among the poorest I've seen when it comes to plenums systems!

In fact, I again met Dr. Jaubert, the originator of the process, at the MACNA in Iowa last year, who again voiced his pleasure with the scientific findings Sam Gamble and I made on the process and the work I've done to eliminate the bad press/incorrect opinions and thoughts about the process over the past decade.

And one of those far outdated and incorrect thoughts was the need for a standpipe/riser tube/opening in the grid. It has been clearly 'proven' that any plenum opening to the bulk water would negate the effectiveness of 'plenum' bed processes and simply turn the whole plenum setup into a common UGF.

Unfortunately, it appears there are still some people not up-to-date on the method, and who, in the best interest of their customers, should update themselves prior to dispensing such outdated/incorrect information. Again, do not connect the space below the grid (plenum) with the bulk water as that cancels out plenum processes and turns the entire bed into nothing more than a common UFG. (again - a proven factor!)

When it comes to sand grain size, research over the past ten years has shown 4 - 5 mm size particle, e.g., crushed coral, to work the best. Anything much smaller, e.g., 1 - 3mm, seems not stay effective for the long haul as it's grains tend to become slightly smaller and/or bind with detritus reducing its pore water size/space between particles. As to the second screen, that was also found not needed unless there were major digging type animals in the aquarium (to prevent a connection with the bulk water!), as sometimes the grains under the screen bind/become concrete-like and need to be broken up. Both of these items have clearly been discussed in my writings for the past ten years!

As for your new FOWLR system, unless a 'real' plenum system is a must (needs to be resigned and depends upon how much sandbed surface is covered), you could go several ways. Remove the second screen where possible, and keep it a simple UGF with working riser tubes. Or remove the sandbed and grid altogether, replace with a bed of about two inches of crushed coral, and go from there.

Hope this helps, Bob



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