Just take your total volume and multiply by 80% (0.8) to account for rock. That's close enough for Zeo, it's even in the guide.
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Originally Posted by Zeo Guide
In order to calculate the required amount of material, take the gross water volume of the total system and subtract approximately 20% for an average decorated tank to come up with the net water volume. Use this net water volume to determine the required amount of ZEOvitŪ.
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