Backyard Aquaponics
A place to discuss aquaponics

Sorry ment 2 & 3.

Oh, okay - not sure how a filter would go with slightly acidic water but I expect it'd be alright. I'm talking about a sort of "Puratap" thing, like the ones used for purifying drinking water.

As far as "plants taking up Zinc" goes, I've done some basic sums and I doubt that the plants alone could remove enough Zinc to make the problem disappear. It would depend on the mass of water entering the system and the mass of plant matter being taken out.

Is it worth the juggle.

Other option is to coat the tank with Sikaflex.. {I think, that's the stuff... C1 or Jaymie would know)...

That's what Jaymie did with her monster tank.... well she did line it with concrete on the bottom as well....

Plants do take up zinc but I suspect that the small amounts of zinc needed by the plants are supplied by the fish feed and/or seasol/maxicrop.

Liner or coating seem like the best options to me.

sikatite (black goop)

Hmmm... I don't know what's more work / more cost - painting the whole inside of my rainwater tank with 3 coats of bitumen (sikatite) or installing a PVC liner? I tend to think the liner is an easier option. Jaymie, do you have any idea roughly how much the sikatite cost? By my reckoning, I'd need at least 150 litres according to the recommended application rates (one primer at 0.5L/m2, plus 2 coats at 1L/m2).

The easiest option of all would be the magnesium anode. I'm still waiting to hear back on a price for that.

My wife has suggested we actually get our water tested for Zinc before we rush out and do anything. Good to have someone sensible in the house.

we got a 20L bucket of sikatite for about $130 2 1/2 years ago. It covered the monster tank 3? times, the inside of the medium corro bed twice, and there is still some left in the bucket, from memory

How big is your "monster" tank?

The Sika website says each of the two coats should be 1mm thick (that's 2 litres per square metre). My rainwater tank is 5m diameter x 2.5m high, which is about 58 square metres if you paint the walls and floor. At $130 per 20 litres, I'm looking at near enough to $800 to complete the job to this specification. For roughly the same price I could get a PVC tank liner. Either way I'd have to empty the tank and would have a lot of work to do.

I'm starting to tend towards the magnesium anode...

the monster is about 3.5m diameter and 1.4m deep. the corro bed is 2m diameter and 60cm deep

Alright, we have our first mortality (apart from the yabbies, which I basically put down to extreme heat, dodgy aquarium and the shock of changing environments).

One goldfish has been found dead. The others are sitting motionless on the bottom of each of the ponds. I realised to my shame that I hadn't pulled the yabby corpses out of the tank so did that, and one had a nasty mould growing on it. I don't know if that could have contaminated the water to the point of harming the fish.

Just tested the water (ranges due to colour variability and my inability to be certain):
pH 7.8 (could be almost as high as 8.0)
Ammonia 0.25 ppm (could be as high as 0.5 ppm)
Nitrite 0 ppm
Nitrate 0 ppm (but could be almost as high as 5 ppm)
Temperature 26 deg C (up from 23 this morning)

I'm worried because this is exactly how the fish behaved in Dad's system - sitting motionless on the bottom, and dying one-by-one. Having started to think about the Zinc issue it's got me quite concerned. Given that Dad's water came from identical rainwater tanks to ours, and we're using his galvanised growbed, this could be the common link...

Have you salted the water yet. I like to run 1 to 2 ppt of sea salt(pool salt).

Also do you have any air running in the system.

No, I haven't salted the system. Actually I'm just reading some articles about water hardness and Zinc toxicity - it seems that by increasing the hardness one can greatly reduce toxicity to Zinc (as well as other things). Looks like I need to harden up the water. Any idea what the difference is between "salinity" and "water hardness" in terms of fish health?


I've been relying on the cascading and splashing into each pond. I haven't got a DO test kit (I think they are available though) so I'm not sure if aeration is a problem. I thought the symptom there would be fish gulping at the surface, not sitting on the bottom?

Ok Jimmy.... at pH 8.0... and water temp 26 degrees... your free ammonia max works out at about 0.4....

Dead yabbies have pushed you into the "lethal" zone... (see attached chart) ...



I'd do an immediate 50% water change... both to lower ammonia... and to lower water temp and pH...

If you can get your pH to around 7.5.... even if your water temp stays the same... your ammonia max doubles to 1...


Salinity has little, if anything to do with hardness... and hardness and "alkalinity" are different

Hardness is a measure of Ca & Mg ions.... Alkalinity is a measure of carbonate buffering....

Harness can be adjusted by adding Gypsum.... a Calcium Sulphate compound... Calcium is beneficial not only to plants but also for your yabbies as they malt...

Epsom Salts.... Magnesium Sulphate... will also adjust your hardness...


I'd be tempted to remove the Goldies into isolation, treat with 1ppt salt... and do several water changes to flush your tank...

With one of the yabbies decomposed... there are possible pathogens in the water that could opportunistically infect your other fish...

Particularly as they're stressed with your current water parameters...