Why is my pH so high in SOME of my tanks?

[redmare]

Ok so here's the short version- my tap water is like 6.6 pH and my 10 gallon betta tank is 7.8 and has been for a while. I don't have anything in there to raise it. Why??

Here's the long version. I currently have 6 tanks running all "new" since early november since I moved across the country and had to upset my existing tanks. Let me give these numbers chart-style for ease of reading..

Tap water

Nov. 3- pH 7.0, GH 8, KH 5

Nov. 30- pH 6.6

Jan. 24- pH 6.6, GH 9, KH 5

10 gallon betta tank- Re-set up November 3. Has a male betta, 6 ember tetras, shrimp and snails. Heated to 80, filtered with sponge, medium-heavily planted, substrate mainly seachem fluorite with some white topfin decorative sand, a few small pebbles from the garden. Very light on water changes, 30% every other week or so, it's hard to keep nitrates above 10 for the plants.

Nov. 5- pH 7.1

Nov. 30- pH 7.8

Jan. 24- pH 8.1, GH 10, KH 4

To help with the whole picture, I'll add what's going on with my other tanks to see if there's a pattern or some other clue.

30 gallon- Set up November 13 with original stock of 5 male guppies and 5 honey gouramis. Heated to 75, filtered with aquaclear 50 and a sponge, tons of insane stem plant growth, substrate half seachem fluorite and half caribsea moon sand. Has many cool large rocks found on the edge of lake Superior and some driftwood. 30% water changes every 7-10 days, nitrate consistently around 20ppm.

Nov 15- pH 7.2, GH 10, KH 6

Nov 30- pH 7.8

Dec 14- added 8 panda cories and a BN pleco. 

Jan 24- pH 7.4, GH 9, KH 3

5 gallon shrimp tank- originally set up back in March, and was the least disturbed by the move. Has a booming colony of cherry shrimp. Heavily planted but with slow growing plants like java fern. Heated to 78, sponge filter, normal aquarium gravel but recently added crushed coral since I moved some guppies in there to have their babies. I do crazy light maintenance on this tank, water change like 10% every 2-3 weeks and it just does its thing. Sadly I didn't record past tests with it.

Jan 24- pH 8.0, GH 18, KH 8

Adult guppy tank- Only been up for about a month. I've been fighting disease in this tank the whole time (but I think I'm winning!) so it's been all over the place parameter wise. Things have been spiking and calming down and I've had salt in for a while and it's a lot. Heated to 75, filtered with aquaclear 30 and large sponge filter, gravel mixed with crushed coral substrate, lightly planted, some wood and rocks from my yard.

Jan 24- pH 7.6, GH 10, KH 7

Baby guppy tank- Set up right after the adult guppy tank. Substrate entirely crushed coral, some plants. 

Jan 24- pH 8.0, GH 11, KH 6

Extra tank- 5 gallon tank I've been cycling with snails for a few weeks now to be used as quarantine or nursery or ~honeymoon suite~ for line breeding. Plain black sand substrate, some crushed coral in the filter, wood, rocks. 

Jan 24- pH 7.6, GH 10, KH 5

So as you can see, it's super inconsistent! Tests are done with API liquid tests, and each water sample was added to the vial immediately before being tested so no water was sitting in the tubes. It makes sense why the tanks with crushed coral would have higher pH than the tap water, but the other ones makes no sense! Especially the betta tank. 

[OceanTruth]

Like you said crushed coral will raise pH, so there's no mystery there. Certain rocks do raise pH levels, so that could be a possible reason in the 30 gallon with large rocks. The betta tank is weird though. Several pebbles I would doubt could alter the parameters of a 10 gallon. Hopefully, someone can spot something in your setups or has a good explanation.

[redmare]

Just now, OceanTruth said:

Like you said crushed coral will raise pH, so there's no mystery there. Certain rocks do raise pH levels, so that could be a possible reason in the 30 gallon with large rocks. The betta tank is weird though. Several pebbles I would doubt could alter the parameters of a 10 gallon. Hopefully, someone can spot something in your setups or has a good explanation.

That's exactly my confusion! It makes sense in EVERY tank except that one!

[OceanTruth]

Is the sand in the Betta tank, Top Fin White Aquatic sand? On the petsmart website it says it may contain minerals that affect pH.

[redmare]

@OceanTruthyes it is- but I used about a quarter cup of it total! It's a little decorative pathway. And for whatever it's worth, the petsmart.com says that but petsmart.ca (where I am) does not.

[OceanTruth]

Just now, redmare said:

@OceanTruthyes it is- but I used about a quarter cup of it total! It's a little decorative pathway. And for whatever it's worth, the petsmart.com says that but petsmart.ca (where I am) does not.

Yeah, I agree a quarter cup seems really negligible.