Measure to Manage: Nutrient Analysis #5


Every so often, I post our nutrient analysis of the aquaponics system water using a Hanna photometer. In the last 6 months, things have been fairly consistent except for the winter months. As the system temperature dropped below 57F, the nitrification slowed somewhat and the nitrite level jumped significantly. I think this was a result of both the cold system temperature slowing down the bacteria and a catfish tank that had fewer fish than I thought. Fish food was collecting in the tank and probably contributing to the nitrite issue as a result. I had been concerned that the catfish were not growing and decided to empty the tank to see what was going on in there. As it turns out, the catfish were almost the same size as when we started last year!

10 month old tiny catfish

Since the catfish tend to stay at the bottom of the tank, I just figured they were hiding, but still growing like the hybrid carp. Unlike the catfish, the carp do surface for food and are a little more action packed when feeding, but a lot of them stay below so I didn’t think it was too suspicious. After talking with a friend with the same fish, he said it took about six months before he started to see the catfish grow. I figured our catfish were similar, but then I looked up when we got them and that’s when I decided that maybe draining the tank would be best to finally figure out what was in there. There were 10 tiny fish inside with a pile of uneaten food that collected in between the airstones. I started with a hundred fingerlings and probably plucked out 20-30 floaters over the last 10 months or so. Any other fish that died must’ve gotten caught in the little “vortex” in between the airstones where the fish food collected. I think this is the case because in the pile there were bits of what looked like broken up fish bones. This unplanned calcium phosphate source was a bit of a surprise. Poor little fish. I knew there some small fish in the catfish tank, but assumed that there were some bigger ones in there as well. I mean, how could they not grow?! I am very curious why they did not grow and will be asking some folks to see if others have experienced this to learn what went wrong. I would like to grow catfish again, but I think I will stick with hybrid carp as my “go to” fish until I’m ready to experiment on the fish side again. I’ll keep an eye on the nitrite level and see if that continues to drop now that the big pile of fish food and tiny bodies is cleaned out. Aquaponics can be less than glamorous when cleaning out a scuzzy tank, I’ll just say that.

Everything else on the analysis seems to be standard for this system. I am looking forward to making some changes to the filtration and see how the system performs with the new gear. More on that topic very soon!

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