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After being filtered by the gravel beds, then the polisher, the water flows through the rafts and then back to the fish tanks.

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Comment by Rob Nash on August 1, 2014 at 6:07pm

Adam, i have to sift the granite to get all the limestone dust and fine bits of granite out of it..

the hydroton, if affordable would be good... i pay $21 / yard for the granite.. delivered. so its a no brainer for us.

the ideal ratio is 2:1 ..but ive seen as little as two 4ft x 12ft (@12" deep, of course) servicing a 1200 gallon fish tank connected to 1200 sft of raft (two 72ft x 8ft) thats something like .08:1

i think it depends on the fish load.. heavy at .5 lbs/gal or light .1 lbs/gal

go heavy on the worms.. as much as one pound of worms per 4x8

Comment by Adam Dusen on August 1, 2014 at 7:58am

Rob, in your estimation, what do you think the minimum ratio (in terms of surface area) for rafts to media would be in order to achieve good filtration of solids and good biofiltration? (It looks like you have about a 2:1?) I ask because where I am located in The Bahamas, media of any kind is extremely expensive so the smaller I can make the media beds, the better.  Here the only real option looks like hydroton as the only rock around is limestone and although one can buy what must be imported granite, it is always contaminated with plenty of limestone.

Comment by Rob Nash on March 30, 2012 at 7:13am

Jonathan, this is a gravel hybrid, all solids are filtered in the gravel, then i run it through a "polisher" to get the last of the finest solids.

Comment by Jonathan Kadish NYC AA Chair on March 24, 2012 at 9:35pm

Rob, do you take any solids out of this system or do you just pump directly to gravel beds from the fish tank?

Comment by Rob Nash on February 14, 2012 at 6:17am

the 36 ft rafts hold 9 boards. @ 8" centers = 72 holes per board. the transpiration of the lettuce took my water use from 100 gal/wk to 300 gal/week (still around 10% total volume)

this was in the 105 degree summer with low RH = 20% outside. exhaust fans running nonstop.

FTR- i was growing kale on the rafts in the summer, NOT lettuce.

Comment by RupertofOZ on February 14, 2012 at 5:57am

How many boards of lettuce per raft... 7 or 8??

Trying to relate it to my hydro days...

Comment by RupertofOZ on February 14, 2012 at 5:54am

Thanks for sharing Rob... interesting, I would have thought the 1000gal would support two rafts also...


I take it the water losses were due to transpiration.... roughly how much, and in what sort of temps/humidity??


I'm intrigued, tomatoes are known to suck water like crazy and throw it straight into the air... but I would have though lettuce would have been relatively insignificant...


PM me if you don't want to put it all out here...

Comment by matthew ferrell on February 14, 2012 at 5:46am

Rob is that 36 inches?  Or is each raft section a 8 x 36 trough with 9, 8' x 4' blue boards?

Comment by Rob Nash on February 14, 2012 at 4:49am

Thanks Rupert. Each raft is 8' x 36' = 288 sft connected to 250sft of gravel grow beds on a 1000 gallon fish tank.

my calculations say i should be able to connect both rafts to one 1000 gallon system, so the total would be 576 rafts + 250 media = 826sft total ...but the water use had me adding water to the fish tank constantly, so i re plumbed it with only one raft per fish tank. Total water volume of each raft is 2000gal.

Comment by RupertofOZ on February 13, 2012 at 11:34pm

Looking fabulous Rob..... how many gravel beds to you have per raft.... total volume???

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