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At the local rock store they have what's called sponage limestone. Has anyone used this type of rock in a tankand would it affect the P/H? I'm looking at placing it in a 125gal Oscar tank.

Don't know that particular name.  Limestone in general will realease calcium into the water, thereby pushing the pH up somewhat.

I'm not sure about the composition of different types of rock or gravel, so I just play it safe and limit myself to the aquarium gravel that is sold at the LFS

This rock wil increase your pH but not by much. Best to stay away from any kind of soft rock, especially Limestone.. Texas Holey rocks  is another one you should avoid. that also increases the hardness then the pH.. find some hard rocks and use ONLY them in your tank..