Sudden Ph Drop, New Cycled Tank


I'm upgrading from my 10 gallon tank to a 40 gallon tank. Before setting up the 40, I ran one of my new filters (I have 2 for the 40) on the 10, along side the existing 10 gal filter, for about 3 weeks. The pH, ammonia, nitrate, and nitrite were all good readings.
When I set up the 40 with the filter that had already been running for few weeks, I checked my water values every day for 5 days, and it looked perfect the whole time. I moved a couple of fish from the old tank, and the water values stayed perfect for 2 days. On the 3rd day after fish, I added plants (java fern, anubias, & Vallisneria). That process caused the loss of a couple of gallons of water, so I refilled with water dechlorinated in the usual way.
That was last night. Today, about 12 hours later, I checked my water values again, and all looks good, except my pH has dropped from around 7-7.2 to about 6.4 . The fish look fine. I checked my tap water, and it is still checking in around 7-7.2. What happened to my tank, and how do I fix it?


Your pH can't account for the additional 30 gallons of water that's it's missing. It might go back up naturally eventually but if it doesn't you might need to buy some crush coral to put in a mesh bag to help it increase up.


it sounds like the plants lowered your PH.
  • Thread Starter


I was thinking that the plants must have dropped the pH, but everything I've read suggests that plants will raise pH ... so I'm confused.
I did leave a couple of plants in the pots, which had some fertilizer ... maybe Osmocote?

Any ideas on how to correct the low pH? Or, will it balance itself out?


Its likely your KH being low. Plants aren’t going to lower your PH.

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