Backyard Aquaponics
A place to discuss aquaponics

No paint inside the growbeds - only painted the outside. I washed the tanks thoroughly though!

The reason for asking is that the zinc in the galvanized tanks might not be very good for fish. Most people who use galvanized tanks line them, coat them, or buy them with a fish safe coating already applied to the inside.

The problem with metals (other than stainless steel) is that aquaponic water tends to be pretty corrosive and can tend to eat away at metals. Some metals are kinda bad for fish like copper and zinc while other metals might not hurt the fish but you wouldn't want to use steel or iron for tanks or pipe where they could rust out and leave your system leaking.

Now to some extent the jury is still out about how bad zinc is for the fish and the situation will be greatly affected by how acidic the system tends to get. Cooking acidic foods in galvanized containers can cause the humans who drink/eat the food to become ill so it is all a matter of degrees.

Very nice, all the best with it.
What ratio of grow beds to fish tank volumes do you have?
What type of fish u going to use?
Should impress the passers by!

Total growbed area = approx 3.9 square metres
GB volume would be about 1300L (which is probably 700L of gravel, 600L of water during flood cycle - at a guess).
Total tank volume = 1450 + 3x400 = approx 2650L, or 2050L during flood cycle.

At this stage, intended fish are silver perch in warm months and rainbow trout in cold months. Also hope to get some mussels and yabbies. I'm intending to err on the side of too small a stocking density, at least until I've got it all hunky-dory.



I'll keep the galv issue in mind. The smaller of the two growbeds came from my Dad's failed system, as did the large tank. I bought a proper liner for the tank as it's underground (didn't want it to rust out) but assumed that the GB would be okay. If I find that I can't keep fish alive, I'll take the gravel out and line the tanks. Fingers crossed it doesn't come to that...

Have added a few pond plants (water lilies and so on), and 10 goldfish (2-3 in each pond) yesterday evening.

Ammonia reading had risen to 0.25 ppm by this morning, but zero NO2 and NO3. Hopefully will start to see the bacteria converting the NH3 now.

just a query about the galv - does anyone have a problem filling their aquaponics systems using rainwater from galv collection/storage tanks?

i'd be interested to know also, at what pH does the zinc mobilise and what other factors are likely to increase problems. Any mad scientists out there know that one?


Does anyone else successfully keep aquatic plants in their fish tanks? I have White water lily ($$$) that i dont want attacked and devoured, also pennywort (not so nice herb if ingested in pregnancy etc!!) ranunculus (water buttercup) and nardoo (native cloverleaf). I have a tonne of duckweed in the backyard pond and want to know if thats useable as a fish food supplement - do they eat it/like it? its a fantastic compost activator so i keep some growing all the time and might it help a system like this??

Duckweed... absoultely.... fish will eat it no problems... and it'll soak up excess ammonia...

Water lillies and other things... not my field... but I know that some people have quite successfully grown them in their tanks...

Don't ask me what varieties though... Faye might know??

Thankyou R.OZ

the fish are already eating the green floaty bits of weed that came with the water plants! i hope thats the biggest thing they can bite.

I'm looking into this. Our 45,000 litre rainwater tank is made of galvanised steel (Bluescope Lysaght "Custom Blue Orb", not aquaplate, not zincalume). I would guess that the mobilisation of Zinc would correspond to the rate of corrosion of the tank itself.

Okay, there is 100g galv per sq. m of sheet metal
(http://www.gaa.com.au/benefits_of_galvanizing/benefits_dohgasl.html

So, if my tank has a surface area of about 60 sq. m, that's 6 kg of Zinc. Suppose my tank took 100 years to fully corrode all of the Zinc, and assume each year the tank fills and empties once. So we're talking 6 kg divided by 4.5 megalitres. That'd be an average concentration of Zinc of around 1 ppm. (This is real "back-of-the-envelope" stuff.)

Unfortunately the recommendation is <0.005 ppm
(http://www.fish.wa.gov.au/docs/pub/WaterQualityDisease/index.php?0408)

So it appears we have a problem in that our rainwater supply is likely to be something in the order of a few hundred times higher than the maximum recommended level of Zinc. Even if my calculations and assumptions are out by a factor of 10, the level of Zinc is probably waaaay higher than recommended for fish.

So two fairly major questions remain:
(1) what does Zinc do to fish?
(2) what does Zinc to to us?

Interestingly you can buy sacrificial magnesium rods to stick in your rainwater tank to lower the corrosion rate. Then you're replacing Zn ions with Mg ions in the water. Hmmm...

I must continue to study this, but it looks as though a 45,000 litre plastic tank liner may be a worthwhile investment. One concern for me is that my Dad's system continually failed, and it ran on rainwater... and his tanks are made from the same material (and by the same people) as mine...

Zinc is good for us and if you start tasting it in your water you have reached recommended levels in you body. ie stop building levels up and start maintaining at current level. its a common test to find out if you need supplementing for nutrition.

unfortunately, human anatomy is my field - not fish anatomy.

Magnesium is also good in low doses - works to prevent leg/muscle cramps, signs of toxicity include diarrhoea.

regular daily doses of zinc = 25mg
Magnesium 100mg

thats for 70kg humans/mammals fish would be a totally different scenario i guess.

Definetly Peskyvore...., whereas we may ingest small doses of zinc,magnesium, copper etc...

Fish swim in it constantly and constantly ingest it, both with their feed and as part of respiration... where it can pass into the bloodstream and (ultimately) the flesh....

Like all "heavy" metals it is cummulative.... and by nature in fish species.... tolerable limits are miniscule in comprison to humans....

Even if you extrapolate the "safe" human level ... 25mg for a 70kg human body...

Most fish we eat would be around the 1kg mark.... so even 1/70th of 25mg would be the equivalent "human" dose....

Consider the above... and you can see... that fish have extremely low tolerances to many metals...

Oh... and what the fish do need... is already incorporated into their (pellet) feed anyway... anything else is excess/excessive...

There seem to be other sources saying that maximum allowable Zinc concentrations in aquaculture are 0.01 ppm, (according to http://www.rirdc.gov.au/reports/Ras/03-012.pdf)

but water from galvanised tanks is claimed to typically be in the order of 2-4 ppm
(http://www.health.qld.gov.au/ph/Documents/ehu/2668.pdf)

So rainwater from a (non-Aquaplate) galv tank is still likely to be 100+ times more concentrated in Zinc than recommended for aquaculture. Hrmmm...

Three options seem to remain with some vague estimated costs:
(1) Tank liner (approx $1000 one-off)
(2) Magnesium sacrificial anodes ($ unknown)
(3) Heavy-metal filtration system ($100-200 installation + $100/year)

I think options 1 and 2 are my favourites, as either one would be a worthwhile investment in the life of the tank by reducing corrosion of the tank walls. I guess magnesium would really just increase the overall hardness of water, and probably even do some good for the fish. But the tank liner seems to be the most direct and foolproof way of removing the source of the Zinc (i.e. the contact between the water and the metal).

Are option 1 & 2 effected with acidic water. Magnesium + acid makes for bubbles.

Re option 1: At the levels of acidity in rainwater, I don't expect a plastic tank liner would be under threat.

Re Option 2: I guess it's the acidity that leads to corrosion/mobilisation of the Zinc in the first place, so the Magnesium would be reacting with the acid in place of Zinc. This would create Hydrogen bubbles in the rainwater tank (where the anode is installed) but I expect they would vent to atmosphere long before the water reached the fish.


Here's a thought, though --- how readily to plants take up Zinc? It'd be kinda neat if they sucked the Zinc right out of the water and the problem disappeared...