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I'm planning a new, 50 gallons (around 190 liters) aquarium and I found very nice looking, red rocks on the shore of the St. Lawrence river, close to where I live. I cleaned them (bleach, brushing) and did multiple tests on them, namely the muratic acid test as well as simply letting the rock sit in water for several weeks and testing the water afterward. All tests came out perfect, the rocks do not seem to affect my water parameters.

However, they crumble (break apart) quite easily. Personally, I do not mind much as it is not too bad and mostly happens through manipulation. At worst, I'll have some pieces spreading around in my sand when time passes. However, I've seen on different sites (including this one) that we should avoid these type of rocks. They say it's bad for fish safety, but I have trouble understanding how my rock could injured my neons, tetras, otocinclus, etc. a way that would be worst than what they face in nature.

Why are crumbling rocks bad for aquarium and is it that big a deal?

The image below shows one of the rocks (if anybody knows what type of rocks that is, please tell it.)

enter image description here

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    Hi, just as a side comment because I'm a bit worried - bleach is extremely toxic to all aquatic life; I would completely avoid using bleach for cleaning anything meant to be used in aquarium setting, I would never use any equipment or decorations cleaned with it even if they seemed to be well flushed with water afterwards. I'm not regarding your main question - let's wait for experts to do it, I'm not an expert.
    – lila
    Commented Jul 13, 2020 at 21:20
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    @lila, thanks for your comment, but without getting into an argument here, actually bleach is super toxic, yes, but only for a short period of time. It breaks down quickly into basic none-toxic compounds. I've used it for many years in many situations, the important things is always to let it dry and sit for at least a day (but I always wait longer) before using what you cleaned with it.
    – Bastien
    Commented Jul 13, 2020 at 22:05
  • Oh okay it's fine, I just wanted to tell you and make sure you're well aware of the potential risks, just in case, because some people don't know about it. It's just my personal opinion - I've read about cases of people accidentally having fatally poisoned their fish and I would never use it nor advise using it because I would go insane worrying about potential risks. And yes, I've read many times on the Internet that people are using it for aquarium cleaning without ill effects - I guess it's okay as long as it is a conscious choice, so that you are aware and know what you are doing.
    – lila
    Commented Jul 13, 2020 at 22:27

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The short answer to this is rock that crumbles or breaks apart easily might be a sign that the rock dissolves easily, so it might leach unwanted minerals into the water.

Rocks in cold climates might freeze apart, so they might be safer - but you really need to know what type of rock it is. In general rocks that break apart easily should be avoided.

The rocks you can find in streams and on beaches, if they are rounded by water, are fairly safe to use in your fishtank. Rounded rocks look nicer in a tank too, in my opinion.

Rocks you find in nature should be disinfected to avoid harmful pests entering your tank. A good way to do this is by boiling the rock for at least 30 minutes and a bit longer if the rock is large (be very careful when you boil the rock as pressure might build up inside the rock and it might explode, causing injury to you or your surroundings).

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Looks like an iron rich (red) shale (mudstone) with poor integrity/strength. What is bad about smaller rocks? I have used tons of flagstone (sandstone type) around my pond and many other various rocks such a "holey limestone" in aquariums. All I ever needed was to squirt them with a garden hose. Except the rock around/in the pond, there is so much I just let the rain clean that. In 70 years of aquariums, I have not yet needed bleach. Wood, like tree stumps, gets the same cleaning.

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