Okay I'll continue here
Should I use sand for substrate?
This is entirely up to you here. They will do fine with gravel, sand, or even a peat or soil substrate.
What type of filtration/aeration should I plan for.
They should have a sponge filter. They come more from still water areas with not a lot of flow, so sponge is all that they need.
I need to read up on them and their natural habitat a bit more but it seems like we are talking about black water?
Is that just heavily tannined water with leafy matter?
Black water is often water deeply stained with tannins. Many habitats (in fact most natural habitats) have some degree of tannins due to the organic nature of, well, nature. "Black" water is usually places with a lot of leaf litter and other natural debris breaking down in the water. Plants are usually scarce or nonexistent outside of floating or emergent plants, due to the water being darker. In nature, there are several kinds of water, most commonly "white water" which is silty murky water (Rio Solimoes for example), "clear water" which has no silt and no tannins (Rio Sucuri is a prime example), and of course, "blackwater" which has heavy tannins (Rio Negro is the most common example replicated in the hobby).
That said, sparkling gourami have a very wide range of habitats naturally, from ditches, swamps, ponds, and of course blackwater creeks/rivers. Some of these will have light tannins, some will have a lot. Some have a decent amount of plants, some do not. So, in this aspect, it's fine which you prefer. If you want a nice densely planted tank, they'll be more than at home in it. Or, you can dabble in with some of the more natural looks using leaf litter, small branches, and other botanical debris. I will address this in your next question.
What kind of leaves or are oak leaves ok, we have a bunch of them around.
Oak is fantastic! You can even use the branches (dead, not live. If they snap easy they're good, if they bend a lot and are green they're no good). Acorn tops make good botanicals to add to the tank as well.
Other good leaves and wood: maple, beech, Birch, poplar, magnolia, aspen, elm, palm, sycamore, Indian almond etc.
Some break down faster than others (maple, birch, aspen, poplar) while others last longer (oak, magnolia, Indian almond). You can, as well, buy botanicals at some shops if you look around.
If you wild collect, has to be brown and dried and must be chemical free (no pesticides). I've experimented with using sycamore seed pods, the firmer ones as botanicals. They're a neat one as well but the larger ones can get messy.
My water has a pretty high gh and high ph.
How hard is it to maintain a tank like that with the water here?
Depending on your GH and pH, you might need to consider using distilled water or RODI water and either mix it with your tap to bring down the GH to a suitable level, or, use full RO/distilled and remineralize it to what you need. A 10 gallon won't be too difficult to maintain this.
Sparkling gourami need a GH below 215ppm, optimally between 50-150ppm, and a pH no higher than 7.8 but you will find 6-7 a comfortable spot for them.
Larger tanks are more expensive and space consuming with RO, but smaller ones not too bad. Peat based soil substrates can also help reduce the pH, but I would advise testing all of this out to find a stable ground and method before getting the fish. Just so you can get the hang of it before something is living in it, in case of any mishaps. Gain some confidence with adjusting the water first
you will need to get a good grasp on how to manage GH, KH, and pH as they're all interconnected when youre playing around with water parameters. Doable, but it is not as straightforward as sticking to fish suitable for your tap water and just maintaining with your tap, but if there's fish you really want to get and want to put the work into, then it's worth it. Just gotta do some homework and figure things out. You'll need a separate reservoir of water prepped ahead of time for water changes to make sure it keeps a stable parameter every time. So it'll take extra steps.
