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I've got some smooth rocks from outside that I'm thinking of adding to my fin tank. I want to make sure they're safe for my aquarium, so how do I properly clean them?

You found the rocks outside, but where exactly? Were they in a field or somewhere that might've had pesticides or chemicals sprayed around? If that's the case, I'd think twice about using them in your tank. If you're still set on using them, I'd soak them in a large bucket of water for a few weeks, changing the water daily to try and leach out any chemicals.

If there's no chance they came into contact with chemicals, a good scrub with a brush in boiling water should do the trick.

Also, what kind of rocks are they? Some types can affect the water's pH and hardness. A simple test is to put a bit of vinegar on them - if it fizzes, they're not safe to use. No reaction and they're probably fine.

Bruno's advice is spot on. When considering adding rocks to your tank, keep a few things in mind. Firstly, the location of where you found them is crucial. If they're from an area with potential pesticide exposure, such as near a field or roadside, it's best to err on the side of caution and avoid using them.

The type of rock is also vital to consider. I always recommend performing the vinegar test to gauge its suitability for your tank. If it reacts with the vinegar, it's likely not ideal for most aquatic life, except perhaps livebearers or cichlids. I personally consult with a geologist friend when I'm unsure about the rock's composition.

Once you've determined the rock's safety, it's time for a thorough cleaning. If the rocks are from a stream, boiling them is a good starting point. I also like to leave them out in the sun for a week or two to ensure any remaining critters are eliminated. A stiff scrub brush is also essential in removing any hitchhiking aquatic life.

I'd advise caution with boiling or baking rocks - I've come across reports of them potentially exfoliating. My personal experience has been with river rock, which I've purchased in bulk from a landscape supply store for a reasonable price. Since it's essentially granite, I've found it to be inert and safe for my tanks, as it doesn't affect the GH, KH, or pH levels. Given that river rocks have been naturally weathered over centuries, I consider them a safe choice. I simply rinse them under the tap and occasionally use a brush to remove any dirt.