ph not dropping?

Hi guys...Ive had my second batch of diy co2 going for about three weeks now. Theres plenty small bubbles floating around the tank so I dont doubt the quantity of co2 is not enough. Last time I tried this the ph settled around 7 after a week. Now its stuck off the chart and isnt dropping. What has changed is that ive removed driftwood and replaced with a rock. Also there are some empty mussel shells I found in a river. I understand shells can be used as a ph buffer but its only a dozen finger sized shells in 100l. Can the rock also be affecting the ph? Does silica gravel affect ph? Ive recently planted a 'pot' with garden soil and topped with gravel. Had my water tested at lfs today at was around 8.Any help and advice would be much appreciated. Dont want my new Hyalobagrus to suffer too much in the alkaline water.

 

Silica gravel is completely inert, meaning it wont affect ph at all.

The shells will definitely buffer ph. The rock you got, did you do a vinegar test on it? Pull it out the tank, let it dry for a while. Then when it's dry pour some vinegar onto the rock. If it bubbles, then it will affect your water parameters, and buffer your ph.

Also, your garden soil... I would remove that immediately. Unless it's remineralized top soil. If not, your garden soil could be too alkaline.

 

Ah cool thanks Sean! Il geterid of the garden soil, my water sprite is busy taking over anyway, as is the vallis. Keen to upgrade my plant species. What product can you recommend to replace the soil with? The rock will have to stay, my catfish love it too much