how to figure out net volume?

[sphelps]

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.

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 ZEOvit®.