The outcome from using feeder blocks depends on using the right blocks and the right size for your tank. After doing a good gravel wash, water change, and filter maintenance, the blocks shouldn't harm your system. Use blocks recommended for your size system and avoid all generic blocks, bulk loose blocks (may already contain mold from lfs humidity), and any blocks containing blood meal. There are blocks available that contain plankton, spirulina, and freeze-dried bloodworms. I've never had any problems when using these blocks.