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PostPosted: Dec 2nd, '18, 02:52 
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Hi everyone. My indoor aquaponics system has been happily growing lettuce for over a year. I dont really post here, but this forum has been my go-to resource for managing the learning curve.

I installed an RO buddy to help with my constantly high pH. After a couple weeks, it came down to a measureable level and my plants were much happier and grew a LOT faster. With everything working , i got lazy and stopped testing things.

I now want to grow a cucumber plant to see if my new LED's are good enough to grow flowering plants. My vine is about 6 feet long and the older leaves are turning yellow and curling up. Then drying out and falling off. New growth looks great.

So i get out the test kit.
Amonia 0
Nitrite 0
Nitrate between 0-1
pH is the same color as the drops. So <= 6.0
KH is <1 (first drop turns yellow)

So i now have the oposite problem i had before i got the RO system. From what i have read, potassium bicarbonate is what i need to fix everything. Buffering pH and fixing what i think looks like a potassium deficiency in my cucumber plant.

Big question is how much to order and how much to add. My total system capacity is about 330 gallons. I have 5, 5" bluegill. 4 4" creek chubs. And half a dozen minnows in a seperate tank. (Random out of the lake. 1 perch and 1 bass for sure)

With low nitrates, do i need to get some more fish also?


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PostPosted: Dec 2nd, '18, 10:29 
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>> how much to order and how much to add

you will use quite a bit so I would buy a 5kg bag (10 pound), that should last a while.
Not sure for US but usually 5kg is a good balance of cost and volume,
smaller is usually costly 10-20kg is usually a lot to have hanging around.

The amount will vary with your system needs and how often you do it,
but it is in the order of half cup to cup for a 200-300 gallon system.
Your best bet is to add around a half a cup to cup regularly.

It is best not to constantly use one product as you tend to get too much of one thing and not enough of another.
The RO will also result in reduced buffering of the system (low KH, GH).
So a good balance would be to add some CaCO3 or Ca(HCO3)2 or agricultural lime.
Crushed (agricultural) dolomite is another option as it adds magnesium.
The amount with vary depending on which you use but again 5kg bags would be ideal to start with
(as with most it will be half cup to cup etc).

That way you can mix them around and get a good spread of minerals.

The trick will be working out what works best for your system, you shouldn't get severe pH changes but keep an eye on pH. Mostly it will take a day or so to change. The big gain will be in your GH and KH if you use a Ca product as well.


Finally as you nitrates and ammonia are low you could easily get away with dosing with a seaweed based or organic liquid fertiliser. You can afford to use one with slightly higher PKH which will be good for flowering plants.

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PostPosted: Dec 3rd, '18, 03:46 
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Thanks for the input. I could only find 1lb bags, so i just ordered 5 of them. I have some cal-mag, but i havent used any since it is for hydroponics and i dont know exactly what chemicals are in it.

I do have some organic "dolomedic lime" that says CaMg(CO3)2 that i use for my tomatoes outside. Perhaps i could pulverise those granules and use that?

I guess if getting some potassium in there and getting the hardness up some doesnt make my plants happier, i will look into some organic fertilisers, or aditional fish. Right now, i am struggling to grow 6 romaine lettuce plants and one cucumber. All of my problems started once the cucumber really started growing. Before that, i was just doing 16 letuce plants at a time and that was going great. Grew so fast they were ready to eat at round 25 days when the seed packet said 35 days.


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PostPosted: Dec 3rd, '18, 19:29 
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yep the cucumber will be hungry - biggest issue is that it will often eat a lot of nutrients for stem & leaf growth but may not flower without K, Mg, P etc.

often got to weigh up whether the advantage is in growing the lettuces
(particularly with the current Romaine lettuce issues in the states) and just simply buying a couple of cucmbers.

Also an option is to do the cucumber in a wicking bed/tube etc and leaving the AP for the lettuce.
The wicking bed will allow you to heavy feed the cuc....

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PostPosted: Dec 3rd, '18, 20:14 
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I top off my tap water (that's been through a dechlorinating filter & also removes heavy metals) with RO water.My tap water has 530+ ppm GH & 340ppm KH all Calcium.I have a 5 litre bottle & put,75% tap 25% RO which = PH 7.3 GH 240ppm (13.5 drops) & KH 120ppm (about 6 drops).I add about 5 litres every week and this keeps it stable.In the fish tank the GH is about the same as the bottle but the KH is 100ppm (about 5.5 drops).Each week before the top up,the fish tank has a DKH of 80ppm (about 4 drops).

Recently I've been using Potassium bicarbonate because I thought there may have been a K deficiency and what I noticed is that bicarbonate doesn't buffer that well,I think you're going to need a carbonate also.As dlf_perth said,a combination of the 3 usual suspects would be needed.

When I add the Potassium bicarbonate to the 5 litre bottle of RO water,I just match the PH of the water in the tank which usually gives a DKH of 4 in the bottle.

I'm going to try a cycle/combination of Calcium carbonate (my tap water method) twice a month,Potassium bicarbonate & Dolomite once each,each month in top up water and then see what happens.

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PostPosted: Dec 4th, '18, 00:21 
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I know sodium bicarbonate can be cooked down to sodium carbonate in a saucepan. I do that to about 30lbs a year for my swimming pool. Sodium bicarbonate is dirt cheap. Sodium Carbonate at the pool store is around $2-3 a pound. I am not a chemist, but it should work the same way for potassium.

Just turn the heat on about what you would fry an egg at, and wait for the co2 to stop offgassing while stirring.


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PostPosted: Dec 5th, '18, 01:50 
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Diesellpower wrote:
I know sodium bicarbonate can be cooked down to sodium carbonate in a saucepan. I do that to about 30lbs a year for my swimming pool. Sodium bicarbonate is dirt cheap. Sodium Carbonate at the pool store is around $2-3 a pound. I am not a chemist, but it should work the same way for potassium.

Just turn the heat on about what you would fry an egg at, and wait for the co2 to stop offgassing while stirring.



I had a quick read about that,and it looks like your theory is right.When you did the sodium,did you weigh it before & after? to see how much actually went.

Also,how exactly did you do it?,just put some sodium bicarbonate in the pan & heated?

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PostPosted: Dec 5th, '18, 06:17 
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Just filled a saucepan about half full and turned on the heat. You can tell when it is offgassing because it is really easy to stir and kindof "silky". It doesnt stir like that when it is cold, or after it is done. I havent weighed it, but to the naked eye, there doesnt appear to be any reduction in volume.

I used a stainless pot tho. I diddnt want to chance any chemical reaction with the non stick cookware. I get in enough trouble as it is using kitchen stuff for what my wife calls "white trash mad science experiments".


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PostPosted: Dec 5th, '18, 19:07 
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"white trash mad science experiments" :lol: I love it.

If you do go ahead and try the potassium bicarbonate convert,can you report back to say if you were successful or not please.

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PostPosted: Dec 8th, '18, 01:06 
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I did a batch yesterday. The stuff i got from a wine making store has a larger grain size then baking soda, so it doesnt really behave the same way in the pot. I couldnt tell when it was "done". So i just cooked it for an hour. Started with 455 grams and ended up with 357 grams.

I wish i knew more about chemistry so i would have an idea of the theoretical mass to expect after 100% conversion.


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PostPosted: Dec 8th, '18, 15:28 
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I think this is the reaction formula

K2CO3 + H2O + CO2 = 2KHCO3 (with high enough temp)

138.205 gms/mol + 18.01 gms/mol + 44.01 gms/mol = 2 X 100.115 gms/mol

Been too long on messing with the calculations for me to remember exactly how this goes but just looking at it I would expect the final mass of potassium bicarbonate to be around 648 grams if 100 % converted. Someone correct me if I messed up or if you know better - It's been a long time since I messed with this sort of chemistry.


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