I've got these for flower vases I got some of Ebay because they were super cheap and these are the problems you MAY encounter...
A... coloration of the water
B... absorption of tank water additives / treatments which who knows what may happen to be honest I don't know
C... When they split they break down and you will have them all through everything incl a choking hazard for fish
D... they sink but over time they start to float.
They look great in vases with flowers or just in a cool shaped clear vase / bottle but personally I definitely would not recommend for tank substrate
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