There are lots of reasons people use galvanized tanks in aquaponics systems. They are cheaper than stainless, they are durable, and they are readily available. Sounds great, right?
Galvanized steel can cause problems in aquaponic systems, however; if you are thinking about using one for your aquaponic system, it could save you some trouble down the road to know what you are getting into.
Galvanized Tanks are Coated in Zinc
Galvanizing is a process where the manufacturer dips the metal tank in a bath of molten zinc at a temperature of around 840°F (449°C). This is usually to prevent corrosion over time and to provide a cheaper alternative to stainless steel.
What’s wrong with a little zinc?
Zinc is a common micronutrient for plants. This leads some people to think it is safe for aquaponics systems. While it is true that plants can tolerate pretty high zinc levels in solution, fish cannot.
Water quality guidelines can be found online if you know where to look. Technical information from ATTRA or the Purdue Agricultural Extension can be recommended. Purdue recommends levels of zinc be kept between .03 and .05 ppm. Most fish will become stressed at .1 to 1 ppm, and will start dying off at 4-8 ppm.According to the American Galvanizers Association, “The biggest factors in the zinc corrosion rate in fresh water are dissolved gasses, hardness/mineral rate, flow rate, and other ions/chlorides, the hardness of the water, and the pH levels.” These factors make it hard to predict exactly how much zinc will be released from your tank.
Can I Just Paint the Tank with Plastic Polymer?
You may have seen examples of someone painting their galvanized tank with a heavyweight epoxy, or a fish-safe polymer of some sort. This has proven successful in some situations and can be considered if it is too late to choose a different tank.
The danger in this option is that new tanks may have an oily layer left over from the manufacturing process, which makes it difficult for the paint to adhere to the surface of the galvanized. Over time, your system may flake or otherwise allow more zinc into the water. If you have an old tank laying around, you may be able to avoid this problem, although using old tanks can lead to other types of problems.
There have been enough mishaps in the aquaponic system startup phase to recommend avoiding this solution. Instead, we recommend finding an alternative to galvanized steel tanks.
Here are some options:
Use a tank liner - If you already have a galvanized tank, and you won’t be investing in a new tank, you might choose to line the tank with something. Heavy-duty pond liner can be used, although it is a little pricey. Also, to get a good fit you will need to cut and join the seams using a sealant.
Build your own - If you are handy, you can build your own tank out of plywood using sealants or a pond liner. You can get creative with materials and with designs. There may be a size limit to how big you can build using your own materials. You may choose a long, trough-like design to help keep things manageable. The difficulty is to build a large enough tank while eliminating leaks and weak points.
Roto Molded Fabrication - Most regions will have at least one or two roto molding (rotational molding) companies nearby. Common products will include septic or holding tanks. Many companies offer specific products for agriculture, although as long as the materials are neutral they will be fine for fish cultivation. Shipping for large tanks can be expensive. Make sure you have a shipping quote locked in before you buy.
IBC Tanks - IBC tanks can be a solution for smaller or medium systems. There are dozens of growers who have found second-hand IBC tanks for under $100. Many sellers will be selling both food grade and non-food-grade IBC tanks. Make sure you ask what was in them. Some sellers pre-rinse their tanks, which can save you some labor.
Using galvanized steel tanks can be a major reason that beginning aquaponic growers lose so many fish when they’re starting their systems. This can lead to confusion over the cause of the die-off, as many a seasoned aquaponic grower knows. If it is not too late, it is recommended avoiding galvanized steel tanks altogether.