Digging into Aquaponics Pt. 2: The Fish

By Sylvia Bernstein

Intro

Welcome back to aquaponics! In my last article we dug into plants. This time, we’ll explore the fish side of the aquaponics equation.

What Types? (top performers)

Before choosing your fish you need to ask yourself a couple questions. First, are these fish for food or for fun?  If your ultimate aim is to eat the fish (tilapia, catfish, perch, etc.) then you will need to have a significantly larger tank than if you selected most ornamental, aquarium fish (koi is the exception). You should have a fish tank that holds at least 100 gallons for edible, or game, fish because they will generally grow to about 10-12” long before they are ready to be harvested.

Second, where will your system be located and what will the temperatures be like during each of the seasons? If you are growing indoors or in a sophisticated, climate-controlled greenhouse, then temperatures will remain fairly steady throughout the year and you could grow just about any type of fish.  But if you are growing outside, in a garage, or in a fairly rudimentary or off-grid greenhouse where temperatures will fluctuate dramatically, you need to be very careful in your selection of fish.  Your best bet is to stick to cold, hardy North American varieties such as bluegill, catfish, yellow perch, koi and hybrid striped bass.

How many?

The number of fish you can grow depends on the type of aquaponics system you have.   In a media-based design, such as our AquaBundance Modular Systems, the grow beds are providing both the biological filtration (bacterial conversion of the ammonia liquid waste into nitrates) AND the solid waste filtration (mechanical trapping of the feces). This is an extremely efficient, cost-effective and low-maintenance way to grow aquaponically, but you have to be very careful not to overload your grow bed filter. We recommend sticking to approximately one pound of fish per 5 – 10 gallons of water when the system is mature. At the start of the system, however, we recommend populating your tank with one actual fingerling-sized fish per five gallons of water both because the fish will be very small (you need a lot of fingerlings to get up to a pound!) and the bacterial bio-filter will be very immature and not robust enough to handle much waste. Over time, you will lose a few of the fingerlings, and if you end up in an over-stocked situation you can either give your fish away to an aquaponic neighbor, or make a new friend by running a “free fish!” ad on Craigslist.

Feeding your fish

What you feed your fish depends on two things – their size and whether they are omnivores or carnivores. Size is pretty easy to understand. The fish needs to be able to get its mouth around the food pellet or flake, and small fish like goldfish, guppies, and fingerlings of any game species need significantly smaller pellets than larger fish.

Next, you need to look at the protein ratio of the feed.  Almost all fish require up to 50% protein in their feed during their earliest stages, starting with “fry” then growing up to “fingerling” size.  The difference between carnivorous and omnivorous fish, however, emerges as they move from fingerling into their juvenile and grow-out stages.  Omnivores, such as tilapia, catfish and bluegill, require far less protein, often tapering down to 30%, as they get older, whereas carnivores, such as trout, bass and perch, still require 40% – 45% protein.

Adult fish will eat approximately 1% of their body weight in feed a day, and juvenile fish up to 7%.  A professionally run aquaculture operation will know exactly how much to feed their fish, and will feed them several times each day to optimize their growth.  This level of accuracy and effort is too much for most home aquaponic gardeners, however, and we use the much more easygoing feeding technique of feeding your fish as much as they will eat in five minutes once or twice a day, depending on how much nitrate you have, or need, in your system.

Water quality testing and maintenance

Fish are totally reliant on you to create a hospitable habitat for them, so there are a few things that you should watch out for to make sure they survive and thrive. First, make sure the temperature of the water is within their healthy ranges. With the exception of cold-water fish like trout, most of the fish I’ve mentioned so far will be happiest, eat the most, and grow the fastest between the low 70s and low 80s Fahrenheit.  The difference with tropical fish, however, is that they won’t survive below a temperature of 60 degrees F, while native American fish will.

Next, make sure that they have plenty of oxygen. The minimum level of oxygen that most fish species can handle is 4 ppm, and most prefer far more than that. Know that as the water temperature increases it becomes harder to keep it oxygenated, so as summer approaches you may want to add an extra aerator to your fish tank.

You should also pay close attention to some water chemistry parameters. Ammonia and nitrite levels in a fully-cycled, mature system should be at or near zero. If they start increasing, that may be a sign that either there is a dead fish in the tank, you have been overfeeding and there is decomposing food at the bottom of the tank, or there is an anaerobic zone somewhere in your grow beds. This is very dangerous for your fish and should be corrected immediately. Also, be sure that your pH does not sink much below 6.0, or that the pH in your system changes rapidly. 

Either condition could be dangerous for your fish.

In conclusion

Fish are the lifeline of an aquaponics system, and certainly the element that makes it the most unique and fun! If you follow these basic guidelines you will be able to enjoy a problem-free relationship with your fish, right up to the point of harvest, should you choose to do so.

Sylvia Bernstein

President, The Aquaponic Source – Try Aquaponics – TheAquaponicSource.com
Author, “Aquaponic Gardening: A Step by Step Guide to Growing Fish and Vegetables Together” –http://aquaponicgardening.com/
Facebook – @TheAquaponicSource
Twitter – @aquapon