My wife uses a Tunze osmolator for top off. She hasn't had any problems with it.
I've also seen people using gravity feed top offs for small tanks. You might want to do a search.
You may also be able to get away with something diy with a float switch and a toms micro-lifter for a small tank.
Also, vesicularia and microsorum are not proper plants for african bioto
pe as they are from SE asia.
has a list of commonly used african plants if you want to be accurate to at least the continent.
Addendum: Didn't realize the size of tank. Might be a little tight for kribs. If your male and female decide to not hit it off, there might be trouble. Make sure you keep an eye on them.
If you are not overly tied to a planted tank, a mated pair of Tanganyika Shell dwellers might be more suitable for that tank. You can make an interesting hardscape, throw in a couple shells and they should be fine.