Hi Arvin,
Tannic acid shouldn't be any problem for most neps, in fact some acidity is probably good for certain low land peat loving species. I usually don't use 100% coco chips but a mixture with inorganic media lik perlite or burn clay to help with drainage. For my growing condition, 100% organic media will be too wet when there's a monsoon season here especially all my neps are sitting outside.
Still, the best thing about coco chip and coco peat is they're CHEAP!!
