I have a Tadpole Madtom, I didn't think thats what it was for a while because all the pics I saw were very dark brown to black, but recently I've seen some lighter colored specimens much more akin to mine. So what I'm wondering is; why? Is color decided by sex? Regional variant? I don't think its environment as he looks the same in a bare bottom tank as over black gravel.

It probably IS environment, plus some genetics. Madtoms and bullheads dont change color as quickly as some other fish. That said, if he's been in a tank with black gravel and adequate shelter for several weeks and is still very pale, then I'd suspect genetics. I dont think there's any sexual color difference.

He was in the tank with black gravel for months, and I guess technically he's still on dark substrate since he can see the navy blue fabric beneath the tank.
Next question then. I've had him for about a year and a half and he's about 3", planetcatfish lists them at a max size of just over 5". Is that common, or a record holder? He'd be pretty awesome with another 2" on him.

... he's about 3", planetcatfish lists them at a max size of just over 5". Is that common, or a record holder? He'd be pretty awesome with another 2" on him.

Several years ago a buddy and I caught a 5" tadpole madtom, due to its massive size there's debate to this day whether it was ID'd properly. (We think so, but we caught it.) I did some "renovation" in my big tank over the weekend and noticed I now have a big fat one at 4", which makes it the second-largest one I've seen. When I catch them here in Florida, they seem to normally top out at 3" as being the more normal size. Other locations may be different. In Texas they get to 48" long and one fried will serve five hungry cowboys at the all-you-can-eat buffet.(*)

HTH

d.d.

(* Everything is bigger in Texas. Especially the tales.)

In my opinion the major factors contributing to your madtom attaining his maximum size are feeding and water changes. If the madtom is well fed and always has clean water he will grow faster and achieve the largest size he possiblibly can, be it 3.5'' or 5''. I would imagine that genetics might play somewhat of a role in determing adult size possibly but am rather unsure about this. Inferior water quality and poor diet will cause his growth to be stunted to some extent and when we are talking about a fish that is only 3'' an 1'' of growth is a significant size change. The better care taken of the fish, the better he will grow. Be careful not to overfeed him though, as this will foul the water and be far worse for him that not feeding him enough. Just my two cents, for what its worth.