Anubias are such slow growers, they don't usually need feeding as such. Which isn't to say feeding isn't good, just that it's not essential for them. Also, commercially farmed anubias are usually grown emersed.. meaning, the leaves are above water. They grow faster that way. The leaf form is slightly different in emersed growth and the new leaves growing underwater will look a bit different, They will be thinner, a bit different in shape.
So long as they have enough light, and they are not demanding of light, they usually do pretty well. If you do find signs of rot in a rhizome, snip off the bad bit with clean scissors or knife. And even a very small chunk of rhizome is capable of regenerating, though it can take months for that to happen. I've a few pieces less than an inch long that simply floated for months before finally starting to sprout, some sprouted four or five new growing points. It will be a couple of years before these wee things are any size at all, but it's interesting to watch them, slow though they are.