I'm New here and want some help....

Ut oh.... those look like Iridescent sharks, but they are actually catfish.

About that 18 gallon tank... check out these pics.
http://www.planetcatfish.com/catelog/pangasii/pangasiu/172_f.php

Here is more info..
http://fish.mongabay.com/species/Pangasius_hypopthalmus.html

their whiskers are very small and can be overlooked when the fish are young.
they do infact seem to be iridescent sharks.
as far as i know they will just keep growing and growing.
i used to love them when i was a kid, but they never lived long enough to grow very big back them because... well... i was a kid.
i dont buy them anymore because i now know how cruel it is to keep them in an aquarium (IMHO anyways)

as for your search for a smaller shark. i dont believe rainbow or red-tailed sharks grow much over 8" but they are agressive towards other fish and eachother.
im not big on many sharks other than balas, but balas grow much too large for even a 55gallon tank and should be kept in groups. i only have one in my 130 gallon tank, but he tends to hang out with my two tinfoil barbs, so i guess hes okay.

They also grow to a nice 2-3 feet long, too. They also need like ten - twenty times the size of tank your offering them.

She deliberately lied to you so you would buy those fish... Or, she was misinformed and tried to pass the ignorance disease to you.

http://badmanstropicalfish.com/your-fish/dangelo_2.jpg

I think the link up there will show about how big the fish'll get to. The pic doesn't do much justice, but I estimate the fish up there is about 1-2 feet long and will grow at least another foot longer.

Howdy.

Kudos to you for researching your options and trying to get loads of infos.

Getting a tank one day, and then fish the next is a very bad idea. You are just about guaranteed to lose your fish that way. There are threads/articles on this site about 'fishless cycling'....Go up to the 'search' button above our posts and type that in, and you'll get the infos.

A fish may stay the right size for the tank it's in....but if it's naturally a bigger fish and you've got it in a smaller tank -- the only reason it doesn't outgrow it is because it's stunted and unhealthy. It's kind of like if you tried to keep your kids feet in baby shoes forever....something's going to give and it ain't natural or pretty when it does.

Apparently you got some really bad advice. Those fish will grow quite large and an 18 gallon aquarium is simply not enough to accomodate even one of them. If you really want those fish, I suggest you get a larger tank. Fish will not grow according to the size of the aquarium. The fish will continue to grow too big for the tank, and even if the growth is stunted by the size of the tank, it will be very unhealthy for the fish, and unhealthy fish aren't any fun to watch.

Yep, like I said, it'll be too big. Heck, it'll outgrow the tank itself, I think! 0_0

I know someone will correct me , but as far as i recall sharks are saltwater, I don't remember any true sharks commonly sold as tropical fish.

The picture looks a LOT like irridescent catfish. You'll be very sorry if you get one. Because
1. you get very attached to them
2. they either get huge or die trying
3. they are a schooling fish ( the more the merrier)
4. they are very skittish ad bounce off the sides/top/ decorations in a tank when startled.
5. If they are not in the book "tank busters",they should be. if you don't weigh down the lid on your tank they are capable of hitting the glass hard enough to knock it off or aside and die on the floor.
Even worse, because of their size poeple tend to house them with huge, aggressive fish. I rescued 1 - once. It got to be well in excess of 10" in my fifty gallon, where it was housed with everything from small rasboras to a pleco, he never bothered the other fish, as long as they were fast enough to get out of his way when he went beserk so they didn't get run over. He competed with my pleco for the spirulana wafer/pellets and shrimp pellets, but never fought, just grabbed the food and ran. It took me from the time he was 6" to find a home for him. Not that many people with non aggressive fish have large enough tanks to house them.

Unless you've got an established filter to put on your new tank, its going to be a while before you can stock it. A lot of the posts both in the freshwater section and the newbie section are about this, reading through them will give you several options.
Either put your tank very close to the sink, or buy a python, water changes are a must, no matter how you go.
I didn't notice where you're from, but the price you were gven for a less that 20 galon tank seems real high to me. Try ebay. It will give you an idea on probably the cheapest you're going to be able to get everything (look through completed auctions for what actually sold & at what price). My daughter got a 75 gallon complete with wood stand & cannister filters, ect. for less than your pet store's 16 gallon. The smaller setups tend to go reasonably. This is just my opinion, but go for footprint in a tank rather than height, unless you want to have anglefish or discus. I.E. 20 gallon long rather than high. If you live near a a large city,March & april are usually the months most clubs have their auctions. They usually sell a least a few complete setups, and their fish, in my experience are very healthy, just don't get both at the same time,
sorry to have gone on & on. Good luck & enjoy