Backyard Aquaponics
A place to discuss aquaponics

Hello everyone,

What should the levels of P and K in the water be?

I don't have anything to measure those levels right now, but if I have a deficiency I could add P and K within safe limits.


Cheers - Jens

Without being able to measure it's all down to guess work and experience,foliar application can take away some of this guess work,you shouldn't get a P deficiency so discount this,your most common will be K then maybe Ca then Mg,foliar applications of these will keep everything sweet. Potassium Cholride,Calcium Cholride,Magnessium Suphate and Iron I used FE EDDHA,would be my choice of sprays. I ran a PH of 7.8 for quite a while and my foliar regime kept everything growing well.

I'm baffled as to what is missing here.

Tomatoes and peppers are looking strong and are growing like gangbusters.

Mint and Basil are yellow, but grow very well.

Cucumber plants grow slowly and look like they have chlorosis.

I have added Iron and Potassium Sulphate, which got the toms and peppers back to strength.

I have bought this micronutrients mix of chelated metals especially for greenhouse growing.

I got 20 liters from the manufacturer for the same as I would pay for 240ML chelated iron alone.

Looking forward to seeing the results of this.

I personally wouldn't add those hydro nutrients until you figure out what the issue is.

How much water in the system? What's your water quality stats? How much iron/potassium sulfate did you add? Most heavy feeder systems are short on calcium, potassium and iron... Do you know what your source water looks like in terms of nutrients?

Hi Eddie,

You can see my system vie the link in the signature.
I have 1500L of water in the system and about 30 goldfish and 10 crayfish.

Right now I have a high level of nitrates ~200ppm as I'm struggling to use it all, so keeping the feeding low and planting more stuff.

I added 250ml iron and 150g Epsom salt over the course of 2 weeks.
Everything improved and the fishes look very happy.

I have now added the micronutrients mix 2 times. 150ml per time each weekend.
Everything is improving again!

City water here is high in calcium.

I guess I should add some potassium also to have all the basics in line.

Cheers - Jens

Sorry, I just saw that I wrote I added potassium sulphate.

That should be magnesium sulphate i.e. Epsom salt.


Cheers - Jens

I wouldn't use the greenhouse mix you got, I know zinc is bad for fish for one. They might be under tolerances now, but if you keep on adding, it won't stay that way.

The easiest way to combat any deficiency is with seaweed extract/kelp powder/or something along those lines, it usually has it all. I'm not sure what you have in Denmark though.

Foliar application of nutrients is much safer for the fish and some say more effective for plant uptake

Hey Colum,

Thanks for spotting the Zinc problem!
I had no idea this was problematic.

I will stop using it right now and just buy pure Iron chelate.

I just read on Bright Agrotech that of all the micronutrients it's only really Iron which we need to add.

So, I'm getting some more Iron chelate and some Potash to up my Potassium levels.

Anyways, the basil and mint is really picking up color now, so I believe that the Iron has helped.

Any more thoughts are welcome!


Cheers - Jens

That is very similar to the issues I am seeing in my small AP setting as well (yellowing/pale leaves, stronger green veins), did the Iron really help in the long run ?

Yeah iron, potassium and calcium usually run low although most folks have high cal/mg in their water so usually just need to add potassium and iron...

Sorry for the long absense!

The iron, Potassium and Magnesium have helped.
Everything was growing like gangbusters into late October, which is unusual this far north.
We have some very mild weather! I tore down the tomatoes in late October too!

Right now we are hovering just around freezing + - 5 degrees C.

I still have lots of mint, which is still growing slowly although it funilly enough doesn't have much taste.

I also have a sort of Chard growing and tasting very good.
Parsley and Cilantro is not growing much, but hanging in there.

So all in all I'm very happy with the system and looking forward to building a bigger one a little closer to spring.


Cheers - Jens

The old leaves of mint lose their flavour, and also it doesn't seem to taste as strong if it has as much water as it wants. Since in aquaponics you can't really let it dry out between waterings to stress it a bit and make it produce more aromatic oils, maybe cut it back (after it warms up a bit) to encourage new growth?

Thanks Mel, that sounds like an idea!

I did suspect that maybe the free access to water might "dilute" the taste.
I must say however, that the taste only went out when it got cold.

I was also contemplating putting them in a separate media growbed where I can water it when I want to.

But first I will cut it right back and then see how the new growth tastes in the spring.

I just had a look at your system today and WOW!
It's hard to look at when it's freezing cold here and it's all I can do to keep the greenhouse from flying off in the winter storms.


Cheers - Jens