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Puzzling on low pH

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I know our local pH can be a bit low but I'm puzzled. I've got a 5 gal, 15 gal and 90 gal. Non-reactive substrates, regular water changes, plants, and supplemented with fertilizer and Equilibrium. The 2 small tanks stay fairly steadily at 6.6-6.8 PH but lately the big tank has been steadily dropping, to like 6.4 or sometimes even lower (hard to tell as the strips and drops don't always agree). I've noticed lately a general stress on the plants and a few fish "flashing" on the substrate, though this may be unrelated. On test trips alkalinity tests lowish but not totally low, and hardness is midrange to slightly high. I have a bag of coral gravel in the filter to try to change things but since I added it a few weeks ago the pH has gotten even lower.

Differences between my big tank and the smaller ones: more plants, more wood, more fish (it's fully stocked). Same water change regime/percentages, same water used, same proportions of fertilizer/Equilibrium.

Any ideas on what is happening, and how/if I should fix it?

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Interesting. I’d have guessed this the other way around: that the 90 gal would be more stable. How often do you perform water changes?

Aquariums can build up humic acid with plant decay, etc. If your KH is low, and the acidity cuts through your buffer, maybe it is just poised for a crash.

How much crushed coral did you add? I think a big tank would require a fair bit for it to be effective. But maybe running your filter through it helps. 

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