I'm getting ready to replant my growbed. I have a 600 gal tank with about 50 hand sized tilapia. My grow bed is 8' long x 2' wide x 1' deep. The water syphons into the grow bed at one end and over a weir (dam) with holes drilled in the top at the other end so the water is only 6" high. The water then drains into a 55 gallon biofilter full of bioballs where the water is pumped back into the main tank at about 600 GPH.
Right now the grow bed is set up for deep water raft, but since there is no prefilter I'm having solid waste buildup in the bottom of the grow bed. I was thinking of putting a drain in the non weir side and shutting off the power once a week to drain the grow tank and solid wastes. I'd be loosing about 50 gallons of water a week.
I was thinking of setting up the grow beds with gravel and continuous flow. The weir would keep the water at about 6" and I could put 8" of pea gravel in the growbed.
I put a barrier in the middle of the growbed and have the water go through the pea gravel first then into the raft system on the other side of the barrier- say 4' on each side. I could add an airstone to the raft side to help make up for lost oxygen on the gravel side.
Which option sounds best?
I would go with gravel since it will take care of a good portion of the filtration as well as supporting plants. If you could make the water flood deeper it would be better but what you describe should work too.
Beware the gravel, do some tests before you buy the gravel since "pea" gravel only describes the size/shape and not the type of rock. You want gravel that will not affect your pH. And slightly larger media may be better too. I've been using 1/2" brown river rock here and that is mostly quartz and doesn't affect pH. If you can get a sample of your proposed gravel and run a few tests before you commit to it you will be better off. First drop some into a glass of vinegar to make sure it doesn't fizz or foam up. If it does, you probably don't want to use it since it probably has too much limestone.
Constant flood/flow is fine for most plants provided you have enough flow to keep things well aerated and mixed up. Some extra aeration to the water before it goes into the bed and perhaps after leaving the bed may help but the BYAP trials comparing timed flood and drain to siphon flood and drain to constant flood seem to show all three systems doing fairly well with only minor variations from one system to the next.
Thanks - I might be able to get the depth up to 11" of gravel and 9" of water, but I would have to lower my stand a bit so the syphon would work properly.
I put in for 10 cubic feet of home depot Vigoro pea gravel for my last grant. Looks like brown river rock and have seen a few hopefull review of it online. I'll check it out when it comes in. Worse case scenario I can use it in my fish tanks in my class room. We do the acid test in our substrate section of marine science so I'll let the students do it.