sand I'd wonder, a lot of times you can get it for free from people tearing down tanks and I'd still wash it out like crazy.
Rock watch out for any treatments and copper. If it's nice tonga branch maybe worth it but ask if tank treated. Keep in mind how you are going to aquascape, I found the branch hard to look nice if you use too much (looks like a brush pile or yard clippings if you use too much, LOL).
Good for you asking first! Do your research before diving in, what may seem like a good deal might not be if it isn't what you need or just not. Double check ratings of items online and think useful lifespan too. It might be worth spending the extra $$ on something new to get a warranty too (some companies will honor a warranty on something bought second hand but a lot will not). Also, there is a reason some things cost more (like tunzes, etc) for their track record and customer service. Plan ahead and you'll save $$ and headaches (and having to get rid of stuff) later

. But then, do as I say not as I do, I'm still learning as I go too

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Oh, and puffers aren't reef safe. Do your research before buying on impulse (I love smart phones I can research on the fly, LOL).