03-14-2012, 11:48 AM
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I just posted in response to your [Egrant] similar question in the other thread, but will repeat and expand a bit here, since you've also raised some other issues.
First on lava rock, I have read that this is calcareous. Calcareous means the rock is composed of minerals like calcium and magnesium which will slowly enter the water, raising the mineral content and thus making the water harder. This may or may not be what you want, depending upon the fish species. The issue with rock is basically the hardness factor with fish.
Some fish need medium hard or harder water; livebearers, rift lake cichlids, and a few others fall into this group. For tanks with these fish, calcareous rock would not normally be a problem, and depending upon the hardness of the source water, may be an advantage. Using gravel or sand made from calcareous rock is usually an advantage.
Many fish are soft water in origin, and while it varies from species to species as to how much hardness they can tolerate, the aim in all aquaria with soft water fish is to keep the water soft, so using rock that is calcareous is not normally advisable. However, if the water is very soft (near-zero hardness), having some calcareous rock is not necessarily a problem as it may help. This is very general, but sufficient for this discussion at this point.
So back to the lava rock, if this is the brownish/reddish porous rock, I believe it is calcareous. The vinegar test may show this if it fizzes; vinegar is a fairly weak acid so if it does not fizz, I would go to a stronger acid. The Regent #2 in the API nitrate test kit works for this, a couple drops on the rock.
River rock is usually inert (= does not affect water chemistry). I have several pebbles of varying sizes of this in some of my tanks, and I bought it from a local landscape/rock supply. The acid test can still be used to make sure.