I have just started up my system and am experiencing pH problems. I am using crushed stone in my grow beds, mainly because clay pebbles are not available here in Nicaragua and they are prohibitively expensive to ship here. I had read that crushed stone sometimes can contain limestone, which will raise the pH. One suggestion that I read about was to test a sample of the crushed stone in vinegar and see if it fizzes or produces at least some bubbles. This is an indication that the stone is unsuitable for aquaponics. I did this and to my relief no bubbles were produced.
Well . . . it seems that I should have done a number of tests because my system is at pH 8.0 or slightly above. (I do not have fish in the system yet.) Each day I bring the pH down to 6.8 and the following day when I test it the pH is back up at 8.0. I have been cycling the system now for about 2 weeks trying to get the pH stabilized at 6.8-7.0 without success.
Does anyone have a suggestion regarding what I can do to resolve the problem? If I keep adding acid to the system will it eventually neutralize whatever limestone is hiding in the crushed stone?
Depending on where your water is sourced from, it may not necessarily be your media that's giving you grief. Where are you getting your water from a well system or municipal water supply?
I don't know anything about Nicaragua, especially when it comes to sourcing fishtank products, but if you can source a KH test kit (API is a popular one and usually sell for less than $10 here in the States), it may clear up a few things. KH (carbonate hardness) is a measure of carbonates and bicarbonates in your water. Carbonates act as a sort of pH buffer, and if there happen to be any in your source water, than they can cause the same symptoms you've been describing, where the acid temporarily drops the water pH, only to be then neutralized by carbonates, causing the pH to swing back up to where it was originally.
In my own personal experience, well water can be notoriously high in carbonates. For example, the well water we have at my place tests at a 8pH and 18dKH and takes 4-5 cups of muriatic acid to bring one 275 gallon IBC tote to a pH of 6.5.
Carbonates aren't super difficult to deal with, but it helps to know about them as a factor so you aren't pulling your hair out
Here are a couple tips:
1) Buy that KH test kit if it's available and reasonably priced. It's a lot less stressful when you can see those levels slowly climbing down, plus you'll know when to ease up on the acid so you don't crash your pH once the carbonates run out
2) Use cheap muriatic acid for bulk adjusting. Ever try eliminating carbonates with phosphoric acid? Don't. Your wallet will thank you.
3) Buy a separate container to treat water in. You're going to need to replace water in your system due to evaporation and such, and it's a lot less stressful to adjust water that doesn't have fish in it.
Hopefully some of that was helpful. I'm sleepy and I tend to ramble
"There is nothing, Sir, too little for so little a creature as man. It is by studying little things that we attain the great art of having as little misery and as much happiness as possible." - Samuel Johnson
I had not considered that it could be the water! We are in the dry season and get our water from a well. During the wet season, we capture rainwater in two large cisterns. I do not have the test for carbonates, but I do have an API testing kit. I will check the pH of the well water tomorrow. I am using muratic acid to bring down the pH. Thanks!
It sounds like you are aware of some external factors (media choices, source water, etc) that can impact pH levels so take my anecdote with a grain of salt. Before our AP system balanced out we were constantly worried about fluctuating pH levels. Adjusting the pH did not seem to have any effect and the levels would climb back into the high 7- low 8 range. My previous traditional hydroponics experience had us so focused on the pH that we were not considering the system as a whole. We stopped daily pH tests and stopped trying to adjust the level. We fed our fish good food and added in plants. After nearly 5 years the system grows healthy plants and fish and the pH is still in the high 7s low 8s. Each system and set of factors is different but if it will support life then it will find its own balance. Good luck!