Rising pH

[Pidge]

Hello,
I have 2 tanks, 1 with just pygmy cories (35L) and the other a community tank with pandas, platies and a bristlenose and a shoal of lampeyes (75L). The pH of my tapwater varies between 7.4 and 7.8. However the pH of my tanks seems to rise to 8 to 8.2 in between water changes. The water is very hard ~200-300ppm total hardness. Both tanks are well planed with lots of bogwood. I use silica sand fine / gravel as the substrate with laterite mixed in. I have a few small pebbles for decoration which I purchased from a reputable aquatics shop. I've also tested these with vinegar for fizzing. The fish are doing well but sometimes look a little distressed immediately after a water change (scratching or clamped fins). This behaviour usually passes within a few hours. I change 10-25% weekly, and make sure I match the temperature and use a chlorine/chloroamine remover (aquaplus). Other parameters are always ok, ammonia and nitrite zero, nitrate less than 20. Does anyone have any thoughts what may be causing the rising pH or any suggestions to this problem? Any thoughts will be greatly appreciated. Thanks.

[worton[pl]]

Hello

rocks and gravel sometimes can caused ph rising. It's easy to check if your gravel can rise ph. Take a handful of gravel from tank put it in small glass container and add vinegar. If it start to bubble you got limestones and they rise your ph. You can also do it with bigger rocks - but you can put few eyedrops of vinegar straight in rock which you took from tank.

Regards.