Getting to know the intricacies of raising fish in a recirculating aquaculture system has been quite an experience thus far. We ended up with 500 Nile tilapia that are all living in one of the two 610 gallon tanks. Surprisingly, we also ended up with some hundred or so common carp in the system.
The carp (photo) weren’t planned, and must have come in as eggs mixed in with some pond plants that were donated to us from a local pond. While carp are a huge pest that outcompetes other wild fish, they are no match for the voracious appetite and frenzied feeding of the tilapa.
As long as we keep all these potentially invasive species locked up in our system until they make their way to the freezer or smoker, then all should be well.
If you’ve read my previous posts, you’ll notice that this one is a pretty big jump ahead in the timeline of our startup. From the time the fish came to now has been a whirlwind couple of months (this is just one of many projects i’m involved in). A short recap is in order:
The fish came as 0.15 oz fingerlings on May 28th (my birthday). They were immediately put into the indoor fry tanks. After a feeding mix up on the second day, one of the tanks was fouled with too much food, and all of the fish were moved into one tank. Yes, that is 500 fish in 100 gallons.
It seemed like the home-made biofilter was working fine at that stocking density for a while. As the fish grew to around 0.25 oz and the feeding increased the total ammonia levels were rising to around 4 ppm each night, and returning to 0.2 by the morning. This was just within tolerable limits, as long as the PH stays low (see un-ionized ammonia
Just to be safe, and to remove the continued buildup of nitrates, we did a 20% water change once per week. That involved a small trick of bringing the PH of our tap water down from 9.2 to 7.2 using a PH buffer chemical.
The fish grew to love me as the hand that feeds them. Every time i walked by the tank they would start churning at the surface, and at feeding times they devoured all of the food that i gave them. At that stage they were getting a fingerling food (from our project partners at Jim’s Online Fish Shop) at 4% of their body weight per day. That worked out to approximately 1/4 cup food, three times / day.
THE BIG MOVE
On June 10th we moved all of the fish down to the main tanks, which were now swinging between 74 – 84 deg. F. Carried down in three buckets, we floated the buckets in the tanks while slowly topping them off with tank water to stabilize the temp and PH. Once the water had equalized, we gave all of the fish a 3 minute salt dip (3% solution) to minimize the risk of parasites (which come off in the salt water).
The fish were then set free, and I haven’t seen them since. I’m down there every day to feed them and maintain the system, but they are very hard to see as brownish fish in the green water, in a blue tank.
I did manage to catch a few yesterday (which involved a very quick net that caused at least 20 fish to jump straight up out of the water and away from my net). They look healthy and are weighing in at an average of 0.85 oz. (the fish pictured here is 3.5″ long). We’re upping their total feed to 1 pound / day, and have been successfully supplementing approx. 25% of that with duckweed that is growing in the filter trays.