Don't know about the fluval, but I have used gravel, sand, and bare bottom with different FH, and one tried all 3 with no difference.
Here's my last FH, when he was in a tank with greyish gravel. Biggest thing with gravel IMO is to go light, do not lay down 3" of gravel as all it will do is become a nitrate factory full of waste. .
Here's my last FH, when he was in a tank with greyish gravel. Biggest thing with gravel IMO is to go light, do not lay down 3" of gravel as all it will do is become a nitrate factory full of waste. .