Um ok well if you want to get an elong you will need to following for a tank. Substrate, your choice, gravel, sand etc. For piranhas you want more than enough filtration as they produce a lot of waste. I use emporer 400's myself for my filtration and an eheim canister filter. Its really up to you though whether you want to have a hob filter or a canister. Just be sure to get more than enough for your tank. For decoration elongs like to have low lighting, a few hiding spots and lots of open rooom to swim. So throw a few pieces of driftwood in there and a few plants maybe on the ends of the tank and leave the middle open. Also another thing they enjoy is a powerhead to create a current for them to swim in. This will also make your fish healthier. I recommend aquaclear powerheads but then again its up to you. Also you will need to heater for the tank, maybe a couple of them for a 90g. You want the tank at a temp between 76 to around 82 degrees all the time. That should about do it for supplies, good luck.
E