So, if you've never heard of it, look up Taal lake in the Philippines. I'm in love with the tawilis! I'm thinking of a (hypothetical) biotope with them, and the two endemic blenny species...but how would I even start?

You could start with a volcano...

...or a really big tank. Tawilis can get to be 6" long, so a school would need lots of room.

I'd be more inclined to go with Kataba (archer fish) and Scatophagus argus in a brackish tank.

Do you have access to the fish? A contact in the Philippines? A contact with an import license? A store that can get the fish?

Next: Do you want to accurately make the biotope, even down to using the correct plant species?
Do you have access to the plants (same questions as for the fish).
Do you know what plant species are found in this lake?
If 'similar' is OK, then it should be easy to select plants that are more commonly available to use.

Sounds like the Tawilis will need a large tank, perhaps a very large tank. They are apparently found in the open areas of the lake. I think I would start thinking along the lines of a 6' long tank, 125 gallons, with a big sump to add volume. Perhaps move up from there- a 6" long fish that thrives in large schools and is highly active would need every bit of that space. Almost a professional aquarium sort of fish, not a hobby level fish.

I cannot find so much about the Blennies or Gobies. In general most Gobies are bottom oriented fish, and can be territorial. If you are already committed to setting up a large enough tank for the Tawilis, then the bottom would have plenty of territory for these fish. Unless you find out differently, I would set up rocks from about 8" on down to sand and gravel, with other things that could have fallen into the water like branches. These would provide territory markers and hiding places for these fish.

For the plants, I would place them across the back, an irregular band perhaps 25% to 30% of the depth (front to back) would be good substrate for the plants, and bring a few patches into some areas between the Goby territories. Leave areas without plants for the Gobies. Plants should not be so tall as to interfere with the movements of the Tawilis, but could be taller against the back glass and sides.

It seems like the tawilis are similar to some of the saltwater fusiliers. Of course, I'd probably also need to look up saltwater sardine care, and start off with sw sardines, since they're probably less expensive.

Diana, like I said, this is a hypothetical sort of thing (for now). I'm in college, and more expenses is something not on my mind...but if I could somehow go collect tawilis, native plants, etc. for my university's biology department, then I'd do that in a flash! The only "contact" I have in the Philippines would be family members...soo probably not.

If I set up a Tawilis tank, then I'd probably just go ahead and ask the exporter for plants from lake taal too. Now that I've read the comments, it seems like this entire project would be a large amount of money, so I might as well just make it as authentic as possible for all the money.

Part of the problem is the current level of pollution in the lake. Make sure your samples are from areas well away from where the highest use of the lake happens.

Many people have a good sized school of much smaller fish, perhaps Neon or Cardinal Tetras, Rasboras or similar. A couple of dozen in a 3' (1 meter) long tank can look really impressive. Most of these fish are under 2" long.

When you double the length of an organism, you are also doubling the height and width. The volume increases by a factor of 2 x 2 x 2 = 8. The oxygen needs and waste (CO2, ammonia) production also increase by a factor of 8.
When the larger fish (Tawilis) are 3 times the length of the sample (2" Cardinals or similar) the waste etc goes up by a factor of 3 x 3 x 3 = 27 times. For EACH FISH.

To maintain a school of a couple of dozen needs a MUCH larger tank than the school of Cardinal Tetras would need.

Highly active fish do better if the tank is at the very least 10x their length. More is better. This is why I suggested you start thinking about a 6' (2 meter) tank for 6" (15cm) fish. Then you add volume for the larger quantity that are socially necessary.

This quickly becomes a proposition for a professional zoo/aquarium sort of place.
While hobby level tanks are sure available 8' to 10' (3 meters) long, the initial investment and ongoing maintenance are not for the faint hearted.

Google some images of
Monterey Bay Aquarium Sardine Tank
Yes, sardines are salt water fish. But apparently Tawilis were, too, and have a similar life style.

Bump: To do this in miniature, different species:

Fresh water Gobies, or Darter Tetras, and White Cloud Mountain Minnows over a pebbled stream, in something like a 20 gallon long, or 40 gallon breeder sized tank. Long and low.
Set up power heads to make a really good flow like this:

http://www.loaches.com/articles/hillstream-loaches-the-specialists-at-life-in-the-fast-lane

Y'know, for such a "small" fish, it seems like this seems more like a MFK tank than anything. And an RC question since they're descended from saltwater fish. Hmm...

Would there be any advantages to using a "raceway" shaped aquarium vs a regular shaped aquarium? I know that with keeping sharks, it's usually recommended to have rounded corners so the sharks don't get stuck. Which is sort of applicable to the question since some of the kept sharks are ram-breathers, and must keep swimming, kind of like the sardines.

Oh hey! I saw the sardine tank at the MBA a few months ago! Although...wouldn't it be different since Taal lake is a ...well, a lake, and not a current-filled ocean?