Aquarium Decoration (Rocks, Plants and Substrates) • Local rocks/ concrete

[Andy88]

I went out for a hike today to find some new aquarium rocks. I found mostly concrete (very obviously), but some I can't quite tell. Partially due to moss growth... any input? I kept one piece that certainly looks natural, but I can't tell 100%. These are some pictures of the area and rocks, as well as the one I'm gonna boil and see what happens.

If they are concrete, I would want to stay clear away from them, right?

[Auballagh]

Yep, those look like good ol' rocks. And yes, I would avoid concrete. The lime used to make those things will definitely drive the PH up in the water of your tank. This could cause problems when you do water changes, as the PH in the water will rise slowly - then snap back down when you dilute it out with fresh add water. That will causes stress on your fish, and your New World Cichlids don't need (or care about) higher PH water anyway...

[Andy88]

Some of the rocks just had such squared edges that I didn't trust them.

If I wanted to test them, I should be able to fill a bucket with water- test pH, then put rocks in, give it a week, test pH again?

[Auballagh]

Hmmm... the PH test thing with the bucket could be an interesting experiment, at least.
As far as the cut/shaped pieces of rock you're finding? That's normal when poking around demolition areas and such. Granite pieces, or even marble at times wind up in those areas along with the usual chunks of concrete, asphalt or other stuff. The cut/shaped pieces of granite were used for wall facing materials, paving stones or all kinds of things. No big deal though, it's still just an interesting shaped chunk of granite.
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NOTE: Inspect all shaped rocks and stones closely for evidence of mortar compounds or cement-boding materials still stuck to them. Those sorts of things are NOT recommended for placement in the aquarium, and must be scraped and cleaned completely from the rock (Or, just don't get collect any rocks with weird stuff on them, y'know?).

[Toby_]

Concrete has aggregate (rocks) mixed in with the sand, cement, fly ash, etc, etc. Like the obvious chunks in your first pic with the rebar in it, you can clearly see the aggregate in the concrete. Also, because of this aggregate, concrete rarely breaks smooth. Though of course it is often formed or cut smooth.

The rocks you are holding are almost certainly a type of sand stone.

Of course, do whatever testing or experiments necessary to feel safe with your choice, but I'm confident you'll be fine using them.