Ph vs Kh and which is more important?

[Connie]

Hello! My husband and I started an aquaponics system in an IBC about 8 weeks ago. We have 40 tilapia in about 215 gal,with sweet potatoes peppers and tomatoes in the GB. All seem to be doing well... Squash and zucchini not so well. I have one question that is driving me nuts! I can't get the Ph and the Kh right at the same time. Source water (well) is Kh 0. Ph is 7.2 or 7.3. If I up the Kh, the Ph flies up. If I add acid (muriatic) it brings down the Ph, but it also brings the Kh back down. Currently the Kh is 2 and the Ph is about 7.2 (API test... Hard for me to read sometimes) I have read that the Kh must be at least 3 for healthy nitrification. Any help would really be appreciated!!!

[Vlad Jovanovic]

Yup the two are totally inter-related. The higher your Kh the higher your pH (up to a point...that point is about pH 8.3 or so...after which Kh can be sky higher but pH can continue to sit at about 8.3 or so). It does seem a bit odd that your well water has a Kh value of dH 0.0 yet a pH of 7.3...I'd expect the pH to be lower, but it is what it is. Water chemistry will vary wildly from location to location...it will even vary, often drastically within the same location depending on the time of year that it is...

Yup conventional wisdom says that (especially while cycling a system) a Kh of about dH 2.5 to dH 3.0 is needed to get nitrification going. It is also during cycling that having a higher pH is advantageous...But if I were you Connie I'd not worry about it too much at this point. At 8 weeks in, your probably out of the woods.

Again, if I were you I wouldn't fret about it. Sit back and enjoy the garden. Test regularly, watch for ammonia and/or nitrite spikes.

Squash and zucchini need a lot of potassium to do well (quite a good amount of magnesium too). That is, if your problems are essential element related. (If they haven't picked up any of the fungal diseases they are prone to)...

There are ways to add potassium while raising your pH if that is what you wish (potassium bicarbonate [KHCO3] or potassium hydroxide [KOH]...but if you don't need your pH raised, but do want to add potassium, then potassium chloride (KCl) would be the way to do it. Epsom salt (MgSO4-7H2O) is a good source of magnesium, so is magnesium chloride (MgCl). None of those chlorides or sulfates will affect your pH any...

[Connie]

Thank you! Very helpful!! I have just retested: Ph 7.2 Kh 2. NO nitrates and ammonia of b/n .5 and 1, up from .25 several days ago. All I can think is that nitrification has slowed or stopped due to low Kh? Should I slowly up the Kh w/ Microbelift's Kh Booster or dolomitic lime and hope the Ph doesn't go up too fast? Tilapia seem to be pretty tolerant! (Cannot find hydrolized lime anywhere locally under any name. And this is an agricultural community!)  Thanks again for your help!

[Vlad Jovanovic]

Ph 7.2 and Kh 2.0 sounds like a more 'common' situation.

You might want to slow down a bit on feeding (not sure how big your bio filter volume is, or the bio-mass of tilapia...40, 100 gram tilapia or 40, 500 gram tilapia...but either way you might cut down a bit on feeding and check for uneaten rotting food in the tank, or a dead fish or two)...in an effort to keep ammonia (NH3/4 and nitrites (NO2)  down.

Don't know what Microbelift Kh booster is, or how much it costs, but anything that is a carbonate/bicarbonate based will raise Kh (and hence pH)...I'm guessing that many of those items are a good deal less expensive than the Microbelift product (i.e. potassium bicarbonate, potassium carbonate, calcium carbonate, magnesium carbonate...dolomitic lime etc)...

The tilapia will be fine as long as your not taking them on too extreme of a pH roller coaster ride...

Hydrated lime (CaOH) isn't going to raise your Kh (since it's alkalinity comes in the form of a hydroxide and not a carbonate/bicarbonate), but it will certainly raise pH in a heat beat, much "quicker" than dolomitic lime, or any other carbonate source...so be prudent as it is very powerful stuff (as are all hydroxides)...

You might find hydrated lime (CaOH) at the grocery store under the name of "pickling lime", or at the hardware store under the name of "builders lime"...

[Connie]

I am trying to err on the side of underfeeding, but I really am feeling in the dark! The 40 Tilapia are now 39... one jumped out. They are not fingerlings... maybe half grown, given to us by a large local tilapia farmer. They are getting one ounce by volume of the pellets twice a day. They make short work of it, so I'm not sure that is enough. But... they seem to be doing great! They get sweet potato vine sometimes. They strip the leaves right off! Poop is couple of inches long. Seems long to me, but I am used to fancy aquarium goldfish.

We have about 215 galleons of water. How much dolomite lime should I put in to raise Kh? I put in maybe 2 cups and got no results. Any wisdom on this?

[Vlad Jovanovic]

Hold off on the Kh raising for now...

So is it correct to deduce that you are using the top of the IBC as a grow bed? And you have only that one grow bed? If so than 40 "half size" tilapia would be about a half pound or so each if we use the sort of common number of 1 lb for a plate sized fish...that would be about 20lbs of fish you have currently.

Using the above numbers it would seem that you may have too many fish for that bio-filter to handle. 

Could you please list todays Ammonia (NH3/4) and Nitrite (NO2) readings...out of curiosity...

"Rules of Thumb" would say that your bio-filter can handle about 12lbs of fish mass.

Carbonates take time to react and dissolve (bicarbonates are much quicker). 2 cups is plenty for now.

[Connie]

Yes one GB, top of IBC, (I think 275 gal IBC.) My husband says the fish are about 4-5 oz apiece. I put in the 2 cups of dolomite lime about 2 weeks ago... And BTW, 31 of these fish were added to our 9 other fish 1 week ago today. That might be the reason for the spike, although I expected that to happen sooner...but I forget these fish are not as "dirty" as goldfish.

My ammonia tester (API) does not distinguish b/n NH3 and NH4 but today the reading is at .5 or even a little lower. NO2 is 0. Kh is 2. I did add  a TBSP of microbelift's booster (that is less than enough to bring the Kh up one pt, but I wanted to move slow.) And yes it is expensive, but the only thing I could find that would actually make a difference. I did not know the lime takes longer than a couple of hours. Was going to put more in today but have not. Ph is steady at 7.2 or slightly lower.

About the ammonia... It has never been absolute 0, always .25. Don't know why. During cycling (fishless) it went up to 4 when I added in the ammonia..The NO2 went up, then all the way down but the ammonia stayed at .25. I have the same situation in my 46 gal goldfish aquarium. Beats me. Must be me. I didn't know I needed a degree in chemistry when I started dealing w/ fish...

Thanks for your time!!

[Vlad Jovanovic]

A wine making shop should have potassium bicarbonate (KHCO3). You can also order it online from Gardenville (sold as an organically approved  natural plant fungicide). One teaspoon of KHCO3 into 13 US Gallons of water should raise Kh roughly 4 dH...and again its very fast acting...

Everybody's NO2 goes up, then plummets down to a nice safe and cozy zero :) ...people refer to that phase in cycling as "the nitrite spike"...So that's perfectly normal.

Your NO3 very well might be zero because of all the heavy feeding plants you have planted. The plants you listed are quite demanding.

Hehe, adding 31 fish can certainly cause a spike in some numbers.

At 4-5 ounces each...you are right at about your bio-filters maximum capacity. So you might need to make a decision soon. Whether to add another grow bed, or get rid of some fish