Sunday, October 9, 2011

Lessons in Water Chemistry Part III; Phone a Friend; and Resumption of Fish Feeding

Since last week marked a full month since I last fed the fish, I decided to seek some expert advice on the nitrate levels (and fish feeding). Back in March I attended the University of the Virgin Islands International Aquaponics and Aquaculture Course. The course is taught by Dr. James Rakocy, who is responsible for most of the work leading to aquaponics as we know it today, and his staff at UVI. Although Jim has retired from UVI, he came back this year to teach most of the week-long course, along with his staff members, Charlie Schultz and Don Bailey. Dr. Wilson Lennard, whose work I sited in a previous blog, also participated by giving a lecture via a remote link-up to Australia.  

In any case, I wrote a quick note to Don Bailey, to see what he thought of the high nitrate levels. Don and the rest of the UVI team are professional scientists, growing tens of thousands of pounds of fish and vegetables annually, using aquaponics, year in and year out, so I value their opinions, to say the least. After he scolded me for starving the fish (sorry Don!), he said nitrate levels are safe up to 500 mg/liter. That's the same as 500 ppm (parts per million). He also said I definitely have a secondary source of ammonia, given that the fish had not eaten for an entire month and that nitrate was still present.

The only obvious source for that ammonia which later gets converted to nitrate, is fish waste and other waste (plant roots, leaves, and other material) trapped in the gravel grow beds. It is likely that the red composting worms are responsible for turning that waste into ammonia, which then gets converted by bacteria into nitrate. The ammonia measurements I took of red composting worm leach ate, which came in very high, support that thesis.

As a result of this interaction, we started feeding the fish again last Tuesday. We started them on a very light diet (1/4 of what they normally got for 2 days, and then 1/2 as much as what they were getting previously), as I was afraid of giving them too much after so much time without food. Also, because of the secondary source of ammonia, I want to make sure I do not add so much food that nitrate levels shoot up beyond the 500ppm level. As a matter of fact, I may leave the feed level at around this rate, or even reduce it, depending on nitrate levels.

Again, my goal is not to grow tons of fish. I am more interested in plant production. Therefore, my goal is to maintain nitrate levels that will result in the maximum amount of plant growth, without maxing out on fish feed and fish growth. I believe that if there is a way to accomplish this, it is by saving fish waste and letting it decompose over time in the gravel grow beds, assisted by red composting worms, rather than by removing it from the system (as is done at UVI and most other places).

In my measurements today, nitrate clocked in at between 100-200ppm. The particular kit I have maxes out at 160ppm, so I took a second reading of a 1:10 dilution for more accuracy, and came up with the 100-200ppm reading that way.

Here's the video for the week.  Besides the water chemistry discussed above, there is also an update on the plants, including cucumbers, tomatos, and lettuce.

PS:  I harvested 6 fully-grown lettuce heads today!  Next week I should have a near raft-full (18) of lettuce ready to take out.  It's getting to be time to upgrade the system.  I'd like to be cranking out 100-200 lettuce heads a week, and have room for other stuff.


  1. Hey Patrick, just curious: what are you going to do with 200 heads of lettuce a week? Are you selling?


  2. Juan, selling would be the plan. Gotta run some numbers first, but if I can keep fish intake down, I think it could work. Why, are you from the government? Do I need like 35 permits for this?

  3. LOL!!! I was wondering if you needed a subsidy or maybe some regulation and then some taxes ;).

    Nah, I was just wondering what one does with 200 heads of lettuce a week!!! (I'm sure you do need some kind of permit for that in CR, there's a permit for everything.)

    There's an organic fair in EscazĂș if you're interested.

  4. Haha. If you're interested, come by sometime. It's in Turrubares about 10 min from Orotina and I'm there almost every weekend. Bring the wife and kids.

  5. Thanks, I thought you were living there. Are you at CDS during the week?

  6. Yep - in the "real world" during the week and on the farm on weekends.

  7. Hey Patrick,
    If the nitrate problem persists, why don't you try a 50% water change? This is a common practice with Koi ponds that don't have a significant plant component in the filtration system.

    Thanks for the posts and videos, I really enjoyed them.

  8. Hey Cap'n Sheeple: I thought of that, but I don't think a water change would have any long-lasting effects. Sure, the levels would drop off, but since I have a source of nitrate production within my system (i.e., it is not just from fish feed) I predict the effect would not last long. Anyway, it's not an issue now because I have it on good authority that nitrate levels can go all the way up to 500ppm.