Water Gremlin removable split shot is one of the finest on the market today. This tin shot is non-toxic and will not harm waterfowl. Removable
With its hydrophilic properties, flies treated with Henry's Sinket quickly break surface tension and rapidly sink where they roll along the bottom as naturally dislodged nymphs would. It's biodegradable too, so no need to worry about harming the water.
A charcoal colored non-toxic, tungsten based product. Just apply to your leader and cast with assurance your nymph will quickly find the bottom.