For other plants you're probably best off raising them up on bricks or something. If it's a water tank it's probably constructed not to rust. You can get black bituminous paint of some sort, I painted the inside of an oildrum once. Second coat was tricky. I think it would probably be OK for wildlife. Always read the label. Depending on the size of the tank, even a lily might be better off raised up. One which grows in relatively deep water may end up with too much spread, if you see what I mean.
