I personally haven't heard good things about concrete, plus it makes your tank heavy as sh*t. We're working on making one outa polystyrene (yes it floats, but siliconed to the inner tank wall)
I've been to 3 different paint stores, talked to about a 1/2 a dozen people there & really no one has recommended anything safe for fish, but I did find a specific paint online that states "fish safe" on it. Theirs also Pond Shield but that's spendy. Also Great Stuff makes a "Pond & Stone" black expandable foam that also states "fish safe" on it. Beckett make similar product. Also used actual aquarium silicone that we picked up at a local fish place...