I got lazy and didn't clean my aquarium sand, what bad can happen?
This is what I used gravel link on Amazon.
So far everything is good.
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As trond points out, the dust is the only real problem you get from non-rinsed sand. Even when I've washed sand you still get a small amount of dust because it's very difficult to remove completely.
However, be aware that all of this dust has to go somewhere. Once your tank has cleared up you'll want check your filter to ensure that the dust isn't clogging it. Most likely, the sponges or filter pads in your filter will have a lot of the dust stuck inside them. Due to the fact the dust is extremely fine, it can really limit flow rate.
Whilst I'm sure you'd pick this up at your monthly filter maintenance, it's definitely worth checking a day or so after you've put in the new substrate just to head off any potential issues.
Most likely nothing will happen, the reason for washing the sand is to get rid of the dust (this is for the sand you buy in bags and labeled aquarium sand); this type of sand have been washed and dried using heat, so it is almost sterile.
But if you make your own sand/gravel, it is very important to clean it to get rid of bacteria and organic waste.
Gravel/sand that has been used before or comes with a used tank needs to be cleaned and sterilized to avoid any diseases in your tank.