i have a 30 gallon tank that has been set up for about 7 months. For months the tank stayed at 7.2( doing all the required water changes of coarse)and starting dropping as expected with an aging tank. At around 6.4 in ph i decided to add crushed coral to stabalize the ph. I added a 1/4 cup and it barely moved. I then added a 1/2 cup and it came to about 6.8. I decided that was reasonable so i left it at that and when i did my 25% water change with water of 7.2 it came up to about 6.9 and by morning it had dropt way down to 6.2-6.3. How has this happend and did i not add enough crushed coral.A couple days later so not to stress the fish. I have added another 1/2 cup \ and history has repeated exactly how i have said before.It came to 6.8 , when i did the water change it drops to 6.2-6.3. So what is happening? Do i need more crushed coral ? There is about 1 1/2 to 2 cups of coral in now.