Getting the right rocks

[Tam]

I am setting up a new freshwater aquarium and was told it was ok to put some Shist rock in it. It is a beautiful blue rock that passes the vinegar test. I put it in a bucket for a week and noted a pH rise from about 7.4 to 7.6. Would it be ok to use this rock for my tank? The fish (at least initially) will be danios, angels, loaches.

 

[cheezysquirrelz]

Look up those fish and find the pH tolerence levels. Make sure all the vinegar is off the rocks!

 

[TankGirl]

In general those fish are very adaptable to pH, and as long as the pH is stable then they will probably be fine. If it raised your pH that much then I would say it is not inert (muriatic acid is a more definitive way to test for this - vinegar is an extremely weak acid) so you will need to watch this closely, and it might not be worth it, if it continues to raise the pH.

 

[dud]

I heard that bottle #1 in the ammonia test kit includes some serious acid that can be effectively used to test the inertness of a rock. Might as well try that if you have the kit already. (and after someone more knowledgeable confirms this).

BTW, I have a pretty rhyolite (igneous rock, inert) that has been tested, boiled and scrubbed, and now a nana is happily living on it!

 

[tropicalman]

It sounds like, at the very least, those rocks will change the hardness in your tank. AS TG says it's stability that you're after. Anything that causes rapid changes in your water chemistry should be avoided as a general rule. I think your bucket test has proven that those particular rocks shouldn't be used.

 

[Lance M.]

Slate is always a good choice. Just make sure it doesn't have any mineral deposits in it.

 

[Tam]

I retested the shist rock for a week in a bucket of tap water and once again the pH rose 0.2, however a control bucket of just tap water did the same thing so something fishy (if you'll pardon the pun) is going on here. Maybe the plastic trash cans I'm using affect the pH. I also did a test using the muriatic acid. It made a piece of limestone foam like a rabid dog ( a very impressive control) but nary a bubble appeared on either my slate or my shist rock.

I think that I will just try this rock in the aquarium and watch the pH over time. Thanks everyone for your help

 

[treedae]

My thought is that as the water sits, CO2 is evaporating out of your water which is what is causing the rise in pH. Reason I'm saying this is b/c I've heard of other people measuring CO2 concentrations and they find them to fall as the water sits after it comes out of the tap. (And since H + HCO3 --> H2O + CO2.....you get a pH increase as the rxn goes toward CO2 b/c it is evaporating out) I'm not sure why the CO2 is so much higher in the tap, but for some reason CO2 pressure in the atmosphere is less outside of the tap than in.

I'd say go ahead with the rock and watch your chemistries closely for a couple of weeks.

 

[TankGirl]

Treedae has it on the head - try the same test by sitting some water out in a glass jar or just a drinking glass over 24 hours and re-test the pH - you might just see a rise in pH.