Correct me if I'm wrong, if you have a media fill GB, it become your Bio filter and among other things lowers your pH level.
What is the size of your GB? Ratio between GB and FT has a direct link on your pH levels.
What are your other readings, ammonia, nitrates, nitrites and temp.?
This maybe way off base, but if your FT to GB ratio is off is it possible to circulate a small potion of your FT water into / out of your pond? Like a constant water exchange, bring pH to your system from your pond and using your Bio filter to benefit your pond as well?
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Welcome Big Mac. I'd start buffering with egg shells as soon as possible. Just roast a few shells in the oven then break them up a bit throw in a nylon or some other form of disposable dunking bag. The shells should stop the ph drop and start to raise it slow enough not to cause harm to the fish. Check the bag every so often to make sure there is still shell left.
I don`t know what species of fish you are working with, some species can handle a pH 5 with no problem. The second and most important is the more base or alkaline the water the more toxic the forms of ammonia are and all are toxic with nitrates being the least. If you are working with tilapia they originally came from.more alkaline water, (not all species of tilapia). Plants are able to uptake nutrients better from 5.3-6.5 then 7 or higher.
Not sure what happened with my profile but it deleted my user profile and my original posts. But here is an update. I was on vacation over the last week so before I left I baked and crumbled almost a dozen egg shells and put them in very thin mens dress socks (like nylons). I put them under the water flow into the grow beds about 2 inches deep in the media. After 8 days I checked pH this morning and system is still at 4.0 still have eggshells left in the bags too. No idea what to do next. I have about a 1-1 ratio GB to FT, hydroton media, I am using yellow perch, I am not using a bio filter. I have changed 1/3 of the water out three times, and even drained the GB to waste and hosed them out to rinse thinking maybe they were anaerobic and creating low pH. I also have no carbonate hardness. havent lost anymore fish for the last week however. Test results are good and were done in a certified lab. I googled the black stock tanks that I have to see if anyone was complaining about low pH with them but didnt find anything. I have been filling the system with water from my pond ran through a sand filter. It has a pH about 7.8 and decent carbonate hardness. I have not measured ammonia or nitrates yet but will try to this week. Any
On the egg shells this is a slow process, which is good. I would not put the sock in the grow bed, not enough water flow. Place it into the FT close to the splash down from your GB.
OK, thinking outside of the box! There are certain rocks, bricks, etc. that you want to avoid using because they will drive your pH upward. You check to see if they have a neutral pH by putting them in vinegar and checking for bubbles. No bubbles, you're good to use them. If you have bubbles that means they will end up driving your pH upward.
Some of you more knowledgeable members need to join me in this thought, since I'm fairly new to AP. Why not place some of these items into your fish tank to help bring the pH level up?
Found something that works so much better than egg shells, I bought crushed Oyster shell for my Japanize Quail. I tested my AP pH last night & found it at 6.0 ppm, so I added a quart of the shells to a strainer bag to place in the FT. 24 hours later the pH was up to 6.8 ppm, hopefully it will buffer off around 7.0 to 7.2. Cheap and fast working...hope it's not raising it too fast, will check it in the morning and remove it. I will then add it back in after work, but will keep a close watch.