Backyard Aquaponics
A place to discuss aquaponics

I too have a a large (300 gal) galvanized tank that has had fish and plants in it with nothing but added well water up for over a year now and I have only had 1 fish die in that time.

I would expect it would have something to do with the age of the galv - if it's old, then the surface has probably fully oxidised and isn't giving off too much zinc any more - not sure about this.

My Dad's galvanised iron system couldn't keep perch or Aust bass alive but a reasonable number of his goldfish survived and grew to a fairly large size (for goldfish) - so it would appear that as a species they are less sensitive to Zinc anyway; on the other and I undestand salmonoids (eg trout) are extremely vulnerable.

Another big point is pH. The lower the pH the more zinc and copper will mobilize into the water. Also zinc and copper become more toxic to fish the lower the pH. If your system pH is high, the zinc might not be as big of a problem as it would be in a system with a pH down below 6.5.

Zinc can become toxic to humans, this is why it is not recommended to use zinc coated vessels to hold acidic foods/liquids for human consumption. Like if some one were to heat apple cider in a galvanized washtub for a fall harvest party or something. Consuming that apple cider could make people quite sick.

As to how toxic using a galvanized tank might be, well there are many variables involved. The reason we need to be so careful with stuff in aquaponics has to do with the fact that it is re-circulating, anything that leaches into the system has the potential to build up quite a bit since we don't normally do major water changes.

If using galvanized tanks, it might be worth investing in doing some testing for zinc levels.

I've noticed that in the "Easy Aquaponics" book there are photos of at least one system that uses galv tanks - I think it's Joel's system. I believe they are "aquaplate" (plastic lined) tanks, so fine for fish, but the article doesn't mention it anywhere.

I think that to avoid "young players" making the same mistake my Dad and I made when starting out, it is CRITICAL that we (as aquaponics evangelists) publicise the zinc toxicity issue. In our case, we saw some pretty photos of galv tanks and growbeds (not knowing anything about aquaplate), then followed the instructions in Joel's original "textbook" and proceeded to watch hundreds and hundreds of dollars worth of fish go belly up.

Had we known about zinc then, we would not have even attempted aquaponics at Dad's place, because the only water supply on the property was rainwater out of a galvanised tank, which must have been laced with zinc as well. Poor fish, never stood a chance...

Agreed - I do the same thing for bathtubs anytime I see someone using them.

I believe most of us do try to warn people every chance we get about metals.
We also warn about using media that affects pH.
And I was even warned about the use of wood in termite territory (still working on replacing my wood/liner beds) I thought It would be ok if the wood wasn't touching the ground.

Perhaps there should be a warnings thread? Perhaps in Useful Information?