It is only really Copella arnoldi that needs much space, even then they often they lay their eggs on the glass, just a few cm above the water line.How much space above the water line do these need?....but is it detrimental to them if you only had a few inches.
I think that would be the most likely explanation.would be the DIY fertiliser capsules I used when I set up the tank (using osmocote-type fertiliser). I planted these deep in the substrate (fine sand) but after adding some new plants, part of the fertiliser might have surfaced.
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