I'm not sure Ian Fuller or @Byron agree with you about substrate.
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On the contrary, there is no doubt that water quality and avoiding bacterial issues is very important, and the type of substrate does impact this too. The issue is providing what the fish "expect" so they will be healthier. If the cories cannot filter feed as in the video below, the substrate is not acceptable. If anyone wants to argue this, do so with the individuals in this hobby who without question know--Ian Fuller, Hans Georg Evers, Luis Tencatt, Marcello Britto, David Sands, etc,. I have always tried to learn from those who have the knowledge, as it seems the wisest path to success and healthy fish.
corydoras ornatus feeding - Search Videos
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