very cool idea Michelle. my only critique is please tell me you aren't going to make a straight stream!! LOL here's an idea for you. habitrail. you know those corrugated tubes they use for rodents? they sell the ones that split in half. and it's pretty thin plastic, which means it can easily be heated and manipulated into a more natural shape. the output of the pump: you can stack a few rocks above the output to give the impression of a freshwater spring. next up, filtration on the pump. poret foam attached to it will work, but it is pricey and you'll only need a small amount. otherwise, you can use a piece of natural sponge, like a loofa, attached to the intake of the pump. as far as substrate level, just grade it, and only plant some marginals along the waterline, just like i did on the left side of my mantella tank
as far as the cocofiber releasing tannins and turning the water brown, you can fill a small nylon sack (either those filter socks, or a stocking foot, or something to that effect) with activated charcoal, and hopefully it will absorb the tannins. you can place it in the pond for a few weeks, or under the false bottom. purigen also works, but once again, pricey. duckweed will be awesome in there, but you'll be scooping it out every day or so....LOL
when you are ready to plant, let me know and i will grab you some sheet moss from my plant place. it's $6/bag and a bag can cover the bottom of a 40B. so figure $12 shipped.ferns, there you hit a chord with me. there are so many!!! check out the 'suzi wong' fern that Black Jungle has. it looks like moss, but is a true fern. otherwise, in that spot, you could plant an oak leaf ficus pumila. tiny ivy like leaves, and it may grow up and out for you. or better yet, i'll get you some moss and a couple small ferns and ship them up when you are ready. i'll just send you pics of the ferns they have while i'm in the store, you can pic the ones out you like....LOL