PH levels rise between water changes
I have an issue with my PH level in a small 54l (I believe its around 14 gallons) tank.
I have this tank set up since October 2015. It's relatively well planted (2 Anubias, 3-4 small Anubias nana, moss, some small Cryptocorynes plants, moss balls). I have 2 medium to small Mopani wood pieces, 2 black rocks and an orange one, bought from a Aquarium shop (should be inert/neutral). The substrate is JBL Volcano Mineral and the sand is JBL Sansibar Dark/Black.
The water in my tank, after a week, rises to around 7,6 PH. The water from the tap, after letting it sit 24h has a PH of 7.0. After a water change it's around 7,3 and it steadily rises until the next change. After i did the setup I noticed it said on the bag of JBL Volcano Mineral that it should not be used together with sand from the JBL Sansibar line, but I saw this to late and I'm not sure what effect it might have.
I believe the JBL Volcano Mineral substrate might have an effect on this, so I put a fistful of it in a bowl, with water from the tap (after it sit for 24h) that had 7.0 PH. After a week the water in the bowl had a PH of 7.2. Might this be the reason, or the main reason? I was thinking of taking it out as I don't have "fussy" plants, but I'm afraid this will not truly solve the problem.
The KH is pretty low in my tank, around 4. I'm not an expert but I know a low KH makes the water buffering capacity low, enabling PH swings. I cannot test DH at the moment, as my test kit does not have a test for this sadly. All other parameters are more or less ok.
I doze EasyLife Carbo ("liquid CO2") and fertilizer also from EasyLife, in accordance to the specifications on the bottles. My tank only has one (but big) Danio female. I used to have a striped Gourami (Colisa Lalia I believe it's named) for almost 2 years but it died recently. I didn't stock the aquarium up for a while, as I noticed this issue and it would be easier to make changes to it without a lot of fish in it.
I want to put a Neon Tetras in it, around 10 of them, but at the moment I don't want to go ahead as I'm afraid I will just hurt and kill the fish (a bit high PH for them + the swings).
If you have any tips for me or any suggestions please let me know.
Thank you in advance,