I think you're erring way too far on the side of caution. Peat moss has 0% nutrients and I'm not sure what "bark fines" are. To me, it sounds like a fancy word for "saw dust". Aquatic plants don't uptake nutrients quite as fast as terrestrial plants - but they still need their share of nitrogen by-products derived from slowly decomposing organic matter. The trick is finding that happy medium where the plants are being fed without poisoning the livestock that makes its home in the same ecosystem. Commercially available potting soils that grow houseplants can be "mineralized" (rinsed and flushed) in order to step down their toxicity under water. My current favorite is Miracle-Gro's "Cactus, Palm and Citrus Potting Mix" , but I'm sure there are others.