Some plants will suck up carbonates (kH or alkalinity) like there's no tommorrow. It sounds to me that this has happened over a period of time and your water change schedule can't replenish it fast enough. What is your kH reading out of the tap?
Another possibility is "old tank syndrom" which involves a complete tear down and or thorough gravel vac. How long has the tank been running?
Either way, the quick way to solve the problem is to bring up the kH by adding 3 tsp of epsom salt/baking soda mixed 2:1. This should raise the kH at least 2 degrees (~35 ppm). Once the kH is corrected, the ph will stabilize. The Gh is not as important.
It would be a good idea to borrow someone's test kits so you can verify your results too. HTH
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