Backyard Aquaponics
A place to discuss aquaponics

Hello, Ryan here from sunny San Diego.

Fairly new to Aquaponics.

I have a problem and am out of ideas, need help !

The problem- I cannot maintain a pH of 6.5+ no matter what I do. ( pH up/sea shells/egg shells)

Most recently did a 60% water change (risky but worked for a couple days..., maintained a pH of 6.5 for a few days.

pH is back at 5.5-6.0.

My system- 2 2x4 grow beds / 100 gallon Rubbermaid trough / hydroton clay pebbles for media. I have a 400GPH pump and good size aerator/air pump.

Fish- 6 small channel cats / 2 small Blue Gills / 6 small gold fish.

Water / chemistry - NITRATES/Ammonia a little high but not crazy.

I have a couple big rocks and a piece of 4" PVC in the tank for shelter/protection for the fish. ( already thought this was the issue and removed both, no joy. now back in)

Any and all help/ideas would be great.

Thank you

Ryan

It seems odd that shell grit (not just a couple of whole shells, which don't have enough surface area) couldn't do the job with so few fish.
I add Calcium Hydroxide (Builders Lime) Ca(OH)2, and Potassium Hydroxide (Caustic Potash) KOH alkalis to neutralise the acidification due the nitrifying bacteria doing their job in my system- the additions are only required when the fish are eating a reasonable amount of food. Shell grit, CaCO3, buffers enough against the acidification when I am only feeding small amounts to fingerlings, less than ~50-80g/day, depending on the food.

With only 400 litres of water you would have to be careful about how much you add of any strong alkali- start small and increase until you have the desired adjustment. I'd start with say 1g of Ca(OH)2 dissolved in water and slowly add that to the system. Give it at least 8 hours to fully mix, depends a bit on how often your pump is running, and it helps if you spread it all around the system when adding, and then check pH.
Don't get any in your eyes or on your skin!

A few lumps of clean limestone in the FT.

I've used limestone a number of times in the past with great success, but recently tried shell grit (in fact I'm still trying it) for the first time and found it very ineffective compared to the limestone.

I will give the calcium hydroxcide a shot.

Thank you !

Ryan

You need some carbonates in the system, they raise, but also buffer the pH, meaning it takes longer for the pH to move. Carbonates are also required for other processes in the system. The reason your pH is low is because there are no, or very little carbonates in the system. Calcium hydroxide doesn't contain carbonates, so while it will temporarily raise the pH, it won't add any buffering capacity to the system.

Also, a balance must be maintained between Calcium and Potassium, too much of one will lock the other out. This is why most people use two pH raising agents, normally Calcium hydroxide and Potassium bicarbonate, and alternate between the two. The Potassium bicarbonate will also help with buffering.

We also are struggling with a low pH and low carbonate hardness but high general hardness. Was just reading about adding sodium bicarbonate (rather than adding yet more calcium) but you mention potassium.

Which is better and if potassium where does one get that from and what quantities do we use? We have a 10,000 litre pool with Silver Perch, and a 2000 AP system and both have the same low pH and kH.

Thanks!


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I would add some lumps of clean limestone to your systems, but make sure you put them somewhere easily accessible, especially for the swimming pool system, because you may need to remove them at a later date.

It'll take a week or two for the limestone to start having an effect, in the meantime I would add some Potassium bicarbonate (much better for your plants than Sodium) at 1x level teaspoon per 1000L. You don't want to move the system pH more than 0.3 in any 24hr period, so maybe only add half the amount then retest the pH and hour or so later.

Thanks for that Mr Damage-
we have always had to keep the kH up (our bore water is naturally low) so since very early on have had either (or both) limestone or shell grit in both the pool and AP. It used to be effective, but it hasn't made any difference now for months. With the GH very high it seems timely to add something other than CaCO3.
Potassium bicarbonate is now on the shopping list!
Thanks for your help
d and m