Which countries and, also, which aquariums?
Do they feed them when the public are in attendance?

Oh yes, they very much still exist in the wild. They flourish in the Amazon River all through South America. There are several species and vary in size. The native people fish for them and eat them and always have.

The natives also swim and play in the river with no thought of danger, because there is no real danger unless the person has a bleeding wound which will attract the fish.

I don’t know of "specific" aquariums that have them, but they ARE in captivity, and can even occasionally be bought in pet stores. I’m pretty sure Sea World in San Diego, California, USA, has them, and possibly the Marine Aquarium near San Francisco.

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I’ve noticed that many large aquariums seem to have 2 or more large holes in the glass at the back of the tank but I don’t understand why they’re there. What are the holes for?

They are for plumbing the tank. Overflows are one more common option, but these holes are also used for in line filters, skimmers, etc, as well.

One must be careful on holes to make sure they do not leak. They also must be careful that the sump or other filter system can support any excess water in the event of a power failure. Check valves are a wise safety net but are not 100% reliable. When people have holes on the bottom of the tank, there is often an overflow divide that would prevent the entire tank from draining in the case that the power went out or something else went wrong.

I’m about to go on a long/exciting trip to this very famous aquarium. I just need to do a little research to get to know how aquariums should improve. I’ve thought of, a I think, 11 or 12? I want to have at least 10 more improvements that aquariums should approve upon of. Thank You!

It depends on the aquarium, I’ve been to amazing aquariums with beautiful displays, healthy fish, and they just take your breath away! I’ve been to others where the average fish shop provides a better showcase.

Things that are nice:
1) Variety
2) Identification and bios on the fish types
3) "Petting" zoo (cucumber, starfish, etc) or other interactive hands-on things
4) Larger walking areas (I hate a cramped aquarium where you cannot even get to the glass to look!)
5) More walk through aquariums (like the shark tunnel ones or the jelly fish ones where the fish surround you)
6) Conveyor belts (been to one with these and they were GREAT!)
7) Ecosystems: don’t just show me the fish, show me where and how they live
Breeding tank: I went to one aquarium that had a thing on how corals grow and fish reproduce. They had coral frags they were growing and eggs they were hatching, they also had a fry tank…very cool!
9) More guides or employees around to answer questions, many people will ask "what kind of fish is that?" and they only have the public there to help them.
10) Information on their system, like a behind the scenes video or information area where they talk about feeding schedules, how they filter the tanks, etc.

I have had aquariums while but none have live plants in them. I planning on setting up a new tank but i know there things about good substrate, amount of light, and co2. I have the light down and DIY co2, problem is now what should i do for the substrate.
I have read about fine gravel, sand, and soil(usually plant soil).
I was wondering which one should i use and how should i set it up.

i have several planted tanks, all with different substrates and all plants do fine … and considering you are going all out with the lighting and co2, i think you can just pick the substrate you like the look of because in my experience plants grow in all mediums … i have the cheapest play sand to the most expensive aquarium sand, ADA aqua soil, and gravel, and the plants do fine in every tank with low light and no co2, only weekly liquid fertilizer …