sand

[Myka]

That would be illegal, and unless you go out to some remote place, the sand will be contaminated by crap that isn't easily cleaned off of it, and you don't want in your tank.

[jslaney]

Also if your sand would be silica based and not calcium carbonate base and would therefore not buffer your water and might contribute to diatomes.

Probably a bad idea.

[RnR]

thanks for the info

[banditpowdercoat]

why is using beach sand illegal? Dad used to use it all the time for my sandbox.