How to correctly set up a new aquarium (so your fish don't die)?

Use all of the water from the old tank to "seed" the bacteria in the new tank. Bag or tote the fish during the transfer in the old water. Dont clean your filters for 2 weeks before the switch and use the old filters on the new tank to jump the cycle.

Ive slammed fish from tank to tank depending on agression, plecs can be very agressive.

So set up new tank, bag/ tote fish in old tank water, transfer all the remaining water from old tank to new, top off new tank with new water, wait for temp to normalize, add fish. Pirhanas are tough river fish they arent temp shocked easily.

I would set the new tank up, let it cycle for a couple of weeks, bag the fish and float them in the new tank taking out 1/4 of the bag water and replacing it with tank water every 15 minutes until they are completely in tank water. Then set them free.
Also, I've found that if you're using charcoal filters, you don't have to do water changes weekly. Actually, I never do water changes and my tanks and fish are very healthy.

when we had the goldfish and other fish I never cleaned the gravel but with the piranhas eating the meats and stuff I like to do it every week to get rid of the food not eaten. I don't want them to eat the old stuff.