Gravel Substrate: Beautiful Colors! Your Red Flint gravel layer will become the home of beneficial bacteria that will break down ammonia form fish urine and waste to nitrites and facilitate the transformation to nitrates. Nitrates are less toxic for your fish. Gravel is also an excellent anchor for the roots of your plants. Overall, Red Flint Aquarium Substrate is just about the best thing you can do for creating unique beauty and balancing of your water chemistry.