Zinc is great in small quantities
Zinc is a plant micro-nutrient – which means that it’s only needed in small quantities – but don’t be mistaken; Zinc, as an important component of many plant enzymes, is crucial to plant growth. Without enough zinc, these enzymes can’t be produced and many essential plant functions slow down or stop altogether, which stunts the plant.
Zinc toxicity is more common than zinc deficiency
Because zinc is included as a micronutrient in most hydroponic solutions and present in fish waste as well as the plating on most fasteners (washers, nuts, bolts, etc.), there is seldom a shortage of zinc in hydroponic or aquaponic systems.
In fact, zinc is more often present in high quantities in aquaponic systems and this can pose a problem. While plants can tolerate pretty high zinc levels in solution fish cannot. In fact, using galvanized steel tanks is one of the major reasons that beginning aquaponic growers lose so many fish when they’re starting their systems. (Zinc and copper are both culprits, but more about copper later.) Toxic levels of zinc will stress or even kill the introduced fish, even when all other variables seem to be just fine, leading to frustration and confusion over the source of fish stress.
Control zinc by using the right equipment
There are a few ways to reduce the zinc in your system. The first way is to stay away from zinc plated and galvanized steel equipment. There are many stainless steel and plastic alternatives that can be used instead.
Covered IBC fish tanksIf a galvanized stock tank is the only option available, the second way to limit zinc in your system is to seal the galvanized steel components. I have used galvanized steel tanks sealed with epoxy paint in the past, with great success. Use a heavy-duty epoxy paint to paint all interior tank surfaces. This separates the galvanized surface from your system solution, and protects your fish from the zinc.
Note that most galvanized or zinc-plated surfaces in biological systems are quickly covered with bio-film, algae and other organic matter. Over time, this forms a natural barrier between the surface of the tank and the solution, and can allow fish to live in galvanized tanks. However, it takes time for these films to develop and if you want to start your system quickly, this isn’t an option.
Zinc can be very helpful to your plants, and won’t hurt your fish at low levels, but watch the components you use carefully, and be wise about what you use, and how you use it.