Scott; Can you expand on substrates to keep in turtle tanks? What kind and how thick? I had a failure with keeping a relatively thin layer (about 2 inches) of “large” aquarium gravel (pebbles) in a 75 gallon tank. Sludge collected under the plastic undergravel plates despite a Magnum 350 intake, two reverse flow powerheads and another powerhead on the undergravel filter plates. The tank also has a large internal Eheim wet/dry filter and a biowheel on the 350 outlet. The water has always stayed crystal clear, but the water quality deteriorated to the point that I lost fish and then had skin problems on the turtles. From what I’ve skimmed on the net in various places, some people may have success in turtle tanks with an undergravel filter with a thicker layer of much finer gravel that doesn’t trap debris and possibly keeps the debris from getting under the undergravel filter plates. But I’m not sure if it works with more than one or two turtles in a tank. By substrate do you mean using it as an undergravel filter in any way or just having something on the bottom for the turtles to dig in and munch on? Todd Scott Howard wrote: