Species of piranha that can fit in a 30 gallon are even harder to come by. Red bellies are fairly common in my area, but the only species I know of in the trade that won't outgrow the tank is an expensive special order. That isn't the case everywhere, though, so good luck to you. If you find any, be sure to post - I don't think I'm the only person who's had eyes out for wimple piranhas.
Anyway, what happened is that you've learned the hard way why the most common bit of advice we give newbies is to stop (or don't start) listening to the pet store. The hobby goes through the bulk of its members, the vast majority of fish store employees, and many fish stores themselves every year, in no small part due to a self-feeding cycle of bad advice and bad experiences.
The shark you mention could be several things, none of them suitable for your tank. Iridescent sharks are beautiful silver striped catfish when you see them in the store, but get upwards of 3-4 feet long, ugly, black, and are highly efficient predators. Bala sharks are also silver, but not striped. They get 12-18 inches long and need to be kept in shoals. There's also red tail black sharks and red finned/rainbow sharks, which are fairly small. However, both species, upon reaching maturity, have personality shifts. They simply won't tolerate a member of their species or something that resembles them in the tank, and some (RTBS are more known for this than rainbow sharks) will begin attacking and potentially killing other species. A betta (fighting fish) is too vulnerable to bullying to be kept with one.
Anyway, as those fish have died, we'll pass on that stuff.
Leaving the tank for three days doesn't even begin the cycle process - there's many methods of cycling. Review this thread for exactly what cycling is, and why leaving the tank empty doesn't do anything:
[URL="http
/www.fishforums.net/content/New-to-t...eady-have-fish/"]http/www.fishforums.net/content/New-to-t...eady-have-fish/[/URL]
If you don't currently have any fish, also review this thread on how to cycle without fish:
[URL="http/www.fishforums.net/content/New-to-t...shless-Cycling/"]http/www.fishforums.net/content/New-to-t...shless-Cycling/[/URL]
The cycle process is hard work for you if you have fish, involving daily (or more) water changes for upwards of a month, and even then can kill fish (directly by ammonia poisoning, or indirectly by poor health and weakened immunities, as yours died) or cause permanent gill damage. Fishless cycling requires a test kit (so does cycling with fish), and a bit of work with an eyedropper.
If you go through a fishless cycle, you can pretty much keep any fish you want, provided you don't overstock the tank, as the tank will be able to cope with the ammonia produced by fish. Here's a thread of good suggestions for a ten gallon tank. Your options are fairly limited, but there's still many available to you:
[URL="http/www.fishforums.net/content/New-to-t...or-A-10-Gallon/"]http/www.fishforums.net/content/New-to-t...or-A-10-Gallon/[/URL]
You've made a good step finding this site - the New to the Hobby section is perused daily by several people who've seen every disaster there is, and will gladly help. If you have any questions, weather about the threads I listed for reference or unanswered questions, don't hesitate to post - unlike a lot of forums, nobody's going to say anything if you repost a question somebody else just asked. Many situations are unique, and individual advice is easy to give and often helps more.
Thanks!! very helpful!! when i setup up my 30 gal tank I will defiantly cycle with no fish. Also what is a good filtration system.in the ten gal i have a whisper carbon system. I want to upgrade with the bigger tank. the pet store tried to sell me a filter that sat on the bottom outside of the tank retailed 189.00 compared to 50 dollers that hung from the bac of the tank. I would like to buy a bigger filter system for more flow is that good?
hay guys and girls I have just bought my son a 10 gal tank and I cycled the tank for about 3 days then we went to the fish store bought a Chinese fighting fish, a couple of snails, and a shark{ not familiar with all the types of fish } it looked cool and my son really wanted it. Well a couple of weeks went by then One fish got ick 
!! 
well went to the pet store and bought some dissolving tab. well I followed the instructions but that one died then one after another they all die except the ghost lobster if you call it that!! will take some pics. Then went back to the store and talked to the guys there and they told me to empty the tank. So I did then did the process over again. Now I'm writing this for some ideas on what kind of fish that are easy to take care of {my son that is! teaching him responsibility on taking caring of something living.} Well anyway I have a 30 gal that i"m going to put piranhas in but they are hard to find! any ideas on that tank also. thanks, hope to get a reply back soon!!!! 
