How could i make a weight that is heavy and small but it doesn't harm the water of my aquarium?

Answers:
Thanks to environmental concerns, you can now buy lead-free fishing weights and split shot in most sporting goods stores.

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I'm assuming you want to sink something. Dense rocks from a local fish store and use super glue gel to attach it to what you want to sink. They usually sell the rock by weight so pick one that has more weight for it's size. Porous rocks will be lighter. Also glass or glass marbles can work but the glue doesn't stick as well. Stay away from adding any metal to your tank as it can corrode and cause problems later.

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I would say to use clean/polished stones, rocks or marbles for weights. Beware of using coral objects %26 shells as they tend to change your PH in the water. I'd also be careful of glues as they can be toxic. If you just get some pretty stones, you could use just the right amt. you need %26 it wouldn't make it too heavy as apposed to using one large rock. Depends on what you're using it for!

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Actually, lead fishing weights are ok to use, if you want. Aquarium stores sell lead wraps, (they look like tie wraps that loaves of bread are tied with, but made of lead), that are sold as "plant anchors"; these are fine to use as well. Also, like the other posters said, rocks or marbles are fine in fishtanks, too.

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A rock? That's what I use lol.

To feed my Pleco, I have to strap some cucumber to a rock with an elastic band. It works! lol