Sterilite tubs are a good way to go for purchased fish (or if you want to sell fish). They stack together easily so they don't take up too much space in the car, and you can put the lids on them to keep fish from jumping out. a bucket for adding/taking water out of the tubs is a good idea as well.
If you're thinking about bringing fish to sell, try posting a list on the ACA group on facebook and wherever else it will be seen. You may be able to pre sell some fish.
Bringing your own booze and snacks is a good way to save some money, as hotel food is usually expensive. I usually go to a grocery store to buy pop/drinks as well.
As other people said, bring bagging supplies along with dechlorinator. It's also a good idea to cut up some of the ammonia remover filter pad to put in your tubs/buckets and bags.
If you're interested in fish and don't buy them right away, write down the room number! You'll be surprised at how many people are selling fish and it's easy to get turned around. If you see something you can't live without, buy it right away. If you want to take a chance, you can wait until Saturday evening, and a lot of people will lower their prices on what they have left to try to move it before the auction.
Peak hours for checking out fish are usually when there isn't a speaker. Other than that, I've seen rooms open well past midnight.
If you like to swim, don't forget your swimsuit as they have a pool.
