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Comment by Amanda Plante on June 21, 2012 at 12:10pm

That's what the product manufacturer for "pH Down" suggested, but two bottles later we're starting to wonder.  But it is a big system (200 gal) and they're really little bottles, so it may work.  Here's a link to the thread: http://aquaponicscommunity.com/forum/topics/ph-mystery .  Thanks for your help!

Comment by Chris Carr on June 21, 2012 at 11:45am

A discussion would have better visibility. Good idea. Since the bounce back is so quick, I can only assume the alkalinity is buffering the pH to 8.0.  Continuing to add acid until all of the buffer is consumed is probably not the best approach, or maybe it is? Ill leave it to the pros to make a suggestion but you may want to test the hardness to see if that is the issue.

Comment by Amanda Plante on June 21, 2012 at 11:30am

pH of the water source is coming out at 7.0.  I'm not sure about the hardness.

I did the vinegar test.  Neither the clay beads and the aquarium pebbles caused any serious bubbles.

The fertilizer is Fox Farm "Grow Big" Hydroponic Liquid Fertilizer: http://foxfarmfertilizer.com/products_liqfert2.html . Until this last application, adding fertilizer had never made such an immediate change to the pH.

Although our TDS was low and it's an organic product, I was initially reluctant to add the product with the fish.  Our system is at a high school.  While the students were out for spring break, we wouldn't have access to take care of the fish.  We ended up taking the fish to our office for a vacation, and I tested out the fertilizer in the system before the break.  After the break the plants looked great.  Come to find out, we had missed a fish who was doing really well despite going hungry for a week and living in the dark in water that had fertilizer.  Ever since then, I've added an ounce of fertilizer every time the TDS drops below 200.  But I would never apply more often than once every two weeks.

This is starting to be a lot of info.  I think I'll move this to a thread, then I'll post a link here.

Comment by Chris Carr on June 21, 2012 at 8:24am

What was the supplemental fertilizer? Also if it is municipal water, could it be that it is really hard? Any idea of what the pH and hardness of the source water is?

Comment by Amanda Plante on June 21, 2012 at 7:33am

Right, it's just clay beads.  It's also worth noting that the pH spike came up very suddenly.  It had been consistently in a range of 7.3-7.6 for two months.  Two weeks ago, I added a little supplemental fertilizer (which I do as needed based on the TDS, never less than two weeks between applications) and the pH jumped to 8.0.  It hasn't come down since, even with pH Down or adding water to compensate for evapotranspiration.

Comment by Chris Carr on June 20, 2012 at 2:47pm

Disregard comment as I see them in other places. 

Comment by Chris Carr on June 20, 2012 at 2:45pm

Is their any media in the system that may contain limestone?