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i have a small aquaponics system running in our test hydroponic greenhouse, the tank is only 300 litres with a flood n drain grow bed above about half the size with a bell siphon installed on a constant cycle. we have very soft water around here and the kH has always been an issue but didnt seem to affect things before. in the last few weeks things have changed we have lost several fish and the water chemisty seems all over the place, the ph was dropping rapidly and the ammonia is rising. i did a half tank change last week with some harder water which raised the kH and i also hung a bag of oyster shells to help keep this stable, however the ph is dropping again and the kH seems to be back to close to zero. and today we have two more deaths.

these are my current readings:

nitrites 0.25-0.5

kH 17ppm

pH 6.0-6.2

ammonia - 0.5

temperature is about 75F

i understand that the low kH is not buffering the pH which is most likely messing about with the conversion of ammonia to nitrites and onto nitrates. but why does the kH drop so much?

when i changed the tank with harder water the kH came out around 100ppm but within a week this has disappeared even with the oyster shells added. so i know the ammonia is too high and the chemisty constantly fluctuating could be the main cause for the fish deaths.

what should i do to fix this problem? travveling to a deifferent water source to fill up containers of hard water doesnt really do much for our carbon footprint. 

i am planning on doing this tomorrow as a friend is bringing me some water with a better kH but i need to top up with our soft water usually so whats the best way to keep the kH up and stop the ph shifting so much?

am i close or am i way off the mark?

help before we have none left.

the only other thing is a lot of the tilapia do not seem to of grown much whilst others have grown a lot and i have noticed some of the larger fish being bullies to the smaller ones, its mainly small fish that have died. but they are all off their food almost completely, apart from one or two of the largest fish. someone mentioned some may have turned female and are getting bullied by the males getting a bit frisky but i dont personally think this is the problem.

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I would recommend alternating some potassium bicarbonate and garden lime (calcium carbonate and magnesium carbonate) as additives instead of hauling water.

I actually like having my pH down a bit lower so I get to use the potassium bicarbonate since my well water is so full of calcium carbonate I've run into problems of too much calcium blocking potassium uptake.  Anyway, the oyster shells are probably not dissolving fast enough.  Some potassium bicarbonate can be gotten from wine making supply shops and I've use a spoon full in each grow bed in my 300 gallon system (the grow beds are 100 gallons each) and you will have to experiment carefully using small amounts of the garden lime since you don't want to raise the pH too fast as that would make the ammonia more toxic.  Probably starting with a 1/4 teaspoon or a pinch of the garden lime dissolved in some system water and poured into the grow bed as it's filling.

The bacteria in aquaponics will consume carbonate in their process and that is what lowers alkalinity or pH.  It is not uncommon for the pH of a system to suddenly drop about the time it gets cycled up.  Now it is possible for a system to run at a lower pH as long as enough carbonates are supplied for the bacteria to work but the pH or alkalinity must be kept farily stable since it if changes too fast it disrupts the bacteria, fish and plants.

that makes a lot of sense cheers, the system has took a long time to mature. we have had poor light levels until this time of year so plants are slow growing and i have had to be very careful to get it to mature slowly over the winter and stocking plants as it goes. i will try adding some patassium bicarbonate then lime, i too like a lower pH as its better for the plants too but this is just dropping too quickly which is not good for them little tilapia. then i have to raise it and it drops again.

do you think i should leave in the oyster shells or get rid? i suppose they aint doing any harm. 

its very frustrating as we are just getting to the best growing season for us.

I assume that if you follow TC's suggestions, your frustrations may quickly end, (this will more than likely be the case, every time til the end of Aquaponic Time...  :)

When your microbes convert 1mg of ammonia to nitrite/nitrate, they use up between (roughly) 7.5 to 8.5 mg of alkalinity in the form of cabonates (as well as about 4.3 mg of Oxygen). 17ppm kH is really, really low if I recall corrctly (it's been a while...we shot for well over 200 for our Oscars...), and while you made no mention of your gH (which will include Magnesium cations as well as Calcuim), it is probably safe to assume that it is low as well?

Potassium BiCarb and Dolomitic (garden) lime (which is roughly half Magnesium Carbonate and half Calcium Carbonate) would help with both.

The real trick with the bicarb and carbonate buffering is that they are slow acting and you have to be careful not to overdo at the same time.  Small amounts often and regularly will be the key.

It will probably take you some time and experimenting to figure out how much and how often will work best to keep your pH between 6.5-6.8.  Now I know many people who can run their systems at a lower pH than this but unless you have a reliable pH meter that you keep in good order, I generally recommend keeping the pH a bit higher since the API test kit only measures down to 6 and if you let it get down to that you don't actually know how low it really is and this is when the pH crash ammonia spiking tends to happen to people.  So I try to keep my pH in the green on the regular API pH test kit.  If the green starts getting a bit too yellow, I go add some of my potassium bicarbonate or some of my hard well water which provides plenty of lime in my case.

Your measurments will probably be something in the order of by the spoon fulls then wait a day and let it mix then test again the next day to see if you need more, you don't want to raise the pH more than .2 per day if you can avoid it.  Eventually you will find out about how much you need to use per week or so to stay within the right range.  Keep in mind that the more feeding you are doing, the more buffering you may need.  Some people have found that their source water hardness will vary depending on season and rainfall too.

i have some potassium bicarbonate in the lab so have just added some of that, we probably have pure calcium carbonate and magnesium carbonate too but i'll wait till the lab person is back cos if one things out of place she'll know and i cant find it easily. i do have loads of pH testers at my disposable since we're a hydroponic retailer so thats not an issue but i was tending to keep mine around 7 because of my dropping pH.

our water is relatively consistent but it comes from reservoirs in the peak district which is mainly limestone so i cant understand why its so low kH.

Do you have any sort of water softener or RO filter in line?


       Much interest of late is going into discovering that nitrification can still happen very well at lower pH which is great news for growing plants with fish.  The key though is at lower pH you have to be far more careful to keep it stable.  But many people doing aquaponics have done very well aiming for a pH between 6.8-7.0 and using the pH of 6.5 as the point at which to take action and add more buffer.

I would only use baking soda in an emergency and prefer to alternate between potassium bicarbonate and my calcium carbonate rich well water. 

Other people have done well with the bag of oyster shells alternating with potassium bicarbonate.  Now sometimes you may need to smash up or stir up the shells if they are not acting strongly enough or add another bag or small additions of dissolved garden lime.

Keep in mind there is a big difference between garden lime and hydrated lime.  The garden lime is mostly calcium carbonate and probably some magnesium carbonate while slaked lime or hydrated lime is calcium hydroxide and it will cause far more drastic effects on pH swings and the big commercial operations use the calcium hydroxide alternated with potassium hydroxide (old fashion lye) but those are the ones that require a special tank for mixing and slowly dispersing the caustic pH adjusters.

TCLynx said:

The garden lime is mostly calcium carbonate and probably some magnesium carbonate


Normal garden lime .. is just Calcium Carbonate...


Dolomite lime.. is Calcuim, Magnesium Carbonate...


And as Magnesium is a trace element, and generally not a prevalent deficiency in AP systems... I would suggest using other buffers for pH adjustment.. as TCL lists...


If you think you need to use Dolomite lime to address a possible deficiency... then do so... and toamtoes can benefit from some Calcium, and Magnesium when fruiting (as well as Potassium).... but in such cases I suggest just actually sprinkling a little directly around the plants in the grow bed... and/or watering it in...

no straight from the mains.

TCLynx said:

Do you have any sort of water softener or RO filter in line?

another fatality today, kH has gone up to around 50 today, gonna raise it slowly and hope it sorts it out, i have noticed though that the larger of the tilapia seem to be becoming aggressive to the smaller ones, they were sold to me all male but i dont know if its the constant bullying thats causing the stress and deaths, it is the smaller to medium sized fish i seem to be losing. although ammonia has gone up slightly more too. ph is 6.8 today so i think the oyster shells and pottassium carbonate are helping.


In commercial tilapia ponds, it is recommended that the fishes be roughly of the same size to prevent "bullying". Separating by size gives the smaller tilapia equal chances to eat and are less stressed. Fingerlings are even segregated into up to 10 different sizes. 


Mature tilapia are even worse bullies.


thats worrying as i dont have anywhere else for them to go and i was just going to keep stocking and removing the larger fish as they grow. is this not really viable?

Joel G said:


In commercial tilapia ponds, it is recommended that the fishes be roughly of the same size to prevent "bullying". Separating by size gives the smaller tilapia equal chances to eat and are less stressed. Fingerlings are even segregated into up to 10 different sizes. 


Mature tilapia are even worse bullies.


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