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No Amonia or Nitrite (well maybe just a little) but 160+ Nitrate

My system completed its cycling a little over 2 months ago.  I am raising a nice amount of lettuce and not much else.  I have 25 tilapia in a 180 gal tank.  The fish are 2.5 to 3 inches in length.  They eat like pigs.


The grow bed is 12 sq feet under 600w Metal Halide lighting run for 18 hours/day.


I have just had an infestation of aphids and white fly and am just now regaining control.  While doing this, I pulled my tomato plants and a very healthy canteloupe that produced nothing but green.


The lettuce is very productive and keeps my wife and myself in salad once per day.


My question is this:  The fish seem healthy, the plants are growing nicely, the nitrate is at 160+.


What is needed if anything to be done?

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Comment by TCLynx on August 22, 2011 at 1:16pm
Yea, actually there is a whole group on the subject of artificial lighting.
Comment by Stephen Eugene Robinson on August 22, 2011 at 12:49pm

Thanks for the comments.  Potassium Supplement is probably right on.  Just yesterday someone looked at the grow bed and said that same thing.  Is anyone else growing under lights?


Comment by TCLynx on August 7, 2011 at 12:42pm

Yep agreed, you need more plants growing to support the tilapia if you want to keep the nitrates lower.  For those 25 fish, I would say you want 25 square feet of plant growing space (12 inches deep preferable but if the ammonia and nitrite stay 0 you might squeek by for a time with shallower beds.)


If all else fails an you have no way of adding more greedy plants to your system, you could perhaps do some water changes and use the high nitrate water to give a real treat to some soil grown plants around the place and in the future stock less fish (as in eat some and don't replace as many of them.)

Comment by RupertofOZ on August 7, 2011 at 9:52am

I'm not an expert on growing under lights Stephen... but I think you might need a mix of wavelength colour to promote flowering/fruiting/seeding in plants under lights...


And you'll almost certainly need a Potassium supplement... at that stage of plant growth...

Comment by Sylvia Bernstein on August 7, 2011 at 9:37am
Looks like you and I were hitting 'send' at about the same time, Rupert!  Good to see you around - you've been missed.
Comment by RupertofOZ on August 7, 2011 at 9:32am

Thanks Sylvia, I was just trying to do the conversions from imperial to metric...

Yep, definitely add another grow bed, especially as the Tilapia grow... the additional grow bed space and vegetables will soak up the nitrates quick smart...

Comment by Sylvia Bernstein on August 7, 2011 at 9:23am

Hi Stephen.  You might also want to post this as a forum topic rather than a blog post - blog posts are usually best for opinion pieces and stories vs. looking for answers and advice from members.  They just are less noticed and people sometimes just don't see them here.

With regard to your question, IMO it sounds like your system is operating well but you might want to consider adding more grow bed space. Your nitrate levels are getting a bit high. Even if your bed was 12" deep (you didn't say so I'm guessing it is less than that) making your bio-filter 90 gallons that is about half of what you will need as your tilapia get bigger - you will ultimately want about a 1:1 volume ratio between your tank (which is stocked just right) and your grow bed.  

Awesome that you and your wife are harvesting salads from it every day!

Comment by RupertofOZ on August 7, 2011 at 9:18am

Plant more plants Stephen...


Not sure about canteloupes, but tomatoes are very heavy nitrate feeders... so having removed them it's probably not surprising to see the nitrate level rise...


Especially as lettuce is also a fairly light nitrate feeder....


Nitrates themselves aren't a problem to fish until they get to levels of about 400-450+... and that's the best reference I've been able to find in many years... and applies to Bluegill...

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