SKIMMER, sterilizer too if fish only tank
and a rubbermaid container or another tank, and filtration/light to set up a quarantine tank. You can set up one for pretty cheap. You can get rubbermaid tubs for 30 bucks for a pretty good one.
Copper for the qt tank and copper test kit. Some peoply say hypo, whichever method you prefer.
Another fish net, catching fish with two nets will be much easier, trust me on that one
vitamin supplement, such as selcon
garlic
Frozen and flake foods, a few different types. Don't just buy brine shrimp and expect everything to be fine.
Seaweed selects or nori from the grocery store if you plan on keeping any herbivores, especially tangs
Buckets, and a hose to siphon water with
towels
substrate, rinse it if using CC
tap water dechlorinator
Airtube comes in handy for adjusting fish as well, but not totally necessary. The plastic spring clamps from the hardware store work pretty good too
Activated Carbon