My pH keeps rising. Why?

Hi everyone. I use rainwater in my bn tank, which goes in at about 6.5 to 6.6pH. However, testing the tank a day or 2 later, and it seems to be in the 7's using API liquid test kit. Hardness is negligible. About 1 degree GH.

Tank contains:
clay cave
2 pieces of driftwood
1 lava rock
2 sponge filters
canister inlet and spray bar
internal power filter
less than 1cm of substrate - gravel mixed with sand

Does anyone have a hypothesis for the pH drifting up? There are ceramic noodles in the canister - Do they ever leach anything. I'm also suspicious of the substrate...
Thanks, Mitch.

If your suspicious of the substrate there is a simple test to determine if your gravel or sand is inert and will not affect the pH.

Remove small samples of the gravel and sand from the tank and drip an acid such as vinegar on the gravel on both samples. If it foams or bubbles it is not inert and will leach carbonates and affect the pH.

Ceramic noodles are made and claimed to be inert.

You could also try the vinegar test on your lava rock as sometimes it's not what it's claimed to be.

indeed, my thoughts would be the 'sand'

unfortunalty most sand is not inert and actually leaches carbonates and calcium into the water which can increase the pH. as the buffering ability of rain water is very low it would only tank a small amount of leaching to raise the pH.

The other option is to head to the local supermarket or hardware store and grab some distilled/demineralised water. test the pH of that then add some of the sand and test evey few hours to see if there is a change. if there is then your sand is not inert and will be the issue. Ceramic noodles are 99% of the time not the issue.

either of these test will help out. if it is the sand then the best replacement (if you still like the sand) is silica sand, the type used in pool filters. This sand it totally inert and cheap as too!

Thanks guys.
I have 2 buckets of rainwater at home at the moment - one with the substrate added and one without. I'll test them this afternoon and see if the pH has changed.
I'll also do the vinegar test.

Results are in:
I checked the pH in both buckets and the rainwater still at 6.4 - same as this morning, and the one with substrate added was 6.8.

So I replaced the substrate with some small gravel/pebbles from another established tank and we'll see how it goes.

Pretty drastic change for only one day.