my current tank stabilises at pH 7.9 and the tap water is pH 7.4, i have just set up a new tank ready to transfer fish and when i tested the water it again has gone up to pH 7.9.... the only things in the new tank are substrate (zambeze sand/fine gravel mix) a few plastic plants and 4 pieces of bogwood. in the old tank are grey medium gravel, 2 polyresin ornaments, a few silk and plastic plants and a juwel 3d background. so why does my water rise up to 7.9 from 7.4? the only other thing in the tank is API stresscoat water conditioner..... my current fish seem fine and its not a worry but i am just wondering why this is happening 
