Rocks changing alkalinity

[Aremihc]

I'm still in the process of setting up my 55gal tank. I recently got 40lbs of stone from a landscape supply place for 5 bucks! I am making caves and such from it. I washed and boiled it, and then put several large chunks in my aquarium, testing the water params first (no fish or anything in there) and no I've tested 1 week later. Here are my results, and I was wondering if they are a prblem, or something to maintain/watch


Pre-Rock Post Rock
pH 6.9-7 6.8
Ammonia 0 0
Nitrite 0 0
Nitrate 0 0
Alkalinity 4(40?) 8(80?) <--- Main question
Hardness 7.5 7.5


What do you think? Thanks

 

[SpeedEuphoria]

I always hear about the vinegar test for rocks, did you try that?

 

[Aremihc]

What's the vinegar test. And I'm more concerned about whether addin alkalinity is going to be bad

 

[Alaris]

The vinegar test: http://www.aquariumadvice.com/forums/f15/marble-natural-stone-104840.html#post887192

Take your stone (dry, not wet), and drop a few drops of vinegar on it and then observe very closely. If you see any small bubbles forming, that tells you the rock is some sort of a carbonate-based rock, which will slowly leach carbonate into your system and raise your pH if you put it in the tank. How much it will raise it depends upon how large the rock is and exactly what type it is. If you have a specialized high pH tank (cichlid tank, Sulawesi shrimp tank, etc.) that extra alkalinity is no biggie. However, if you have a small tank you are trying to keep at acidic or near-neutral pH, having a big chunk of limestone or similar rock can potentially bump your pH significantly higher than you might like it.

Alternative method: http://www.aquariumadvice.com/forums/f15/marble-natural-stone-104840.html#post887219

Sometimes the vinegar test isn't 100% accurate. If the amount of carbonate is too low it may not react witht he vinegar but still affect your KH/pH. If there are strips of different stone you don't test all areas, one of the ones that wasn't tested could affect the water as well. A much more sure fire way to test is as follows.
Fill a bucket with water and either let it sit for 24 hours or aerate it for an hour to get rid of any dissolved gasses. Test for pH/KH/GH. Add the rocks and test again a few days later. Any change in the test results indicates that the rocks will affect the water chemistry.