I have this very tank as well, good luck!
Read through this whole thread and I see you frequent Petco a lot. In case you didn't know they have a $1/gallon sale on their fish tanks all the time. Pick up a 10 gallon Aqueon for $10 and dedicate it to be your hospital/quarantine tank. Heck, that tank is only $15 normally so I'd get it either way. (I got a 10 gallon for fish QT and a 5.5 gallon for inverts QT. Total cost was $30 for the two tanks.)
Of course you will also need to add to that: HOB filter, heater, ammonia badge, thermometer, PVC pipes for hiding and possibly a powerhead or sponge filter depending on what you're treating and what fish you have in there.
All told my fish quarantine hardware consists of—
- 10 gallon tank (from Petco), $10-$15
- glass lid (for this tank, from Petco), $10-$15
- heater, $15-$20
- HOB filter, $20-40
- thermostat, $7
- ammonia badge, $3
- PVC pipe from Home Depot, $1-$3
- I'm assuming you already have the salt to spin up this tank
If you budget ~$100 that should get you completely set up for when your fish get sick.
Dump in some Bio-Spira ($15) for instant life support and you're ready to go.
As for meds... well since you've already bought the fish and introduced them to your DT I'd say your approach is more "reactive" than "proactive", meaning as you identify a disease you'll be going out to your LFS or Petco to pick up the meds to treat it. But it wouldn't hurt to start stocking up on the most common meds now for this inevitability. Copper, metro, Prazi, General Cure, Focus, and on and on. Pick one up here and there to spread the costs out over time.
But please, please, please do not think you have dodged a bullet. Even if you have, this scare should be telling you that you need to get ahead of things and be prepared for an outbreak. You mentioned you had nowhere to go with your gramma when you saw signs of ich. Where are your fish going to go when this happens again? You will have to leave your DT fallow for 11 weeks. Don't be handcuffed again. When you see signs of this recurring, and it sounds like you will, have that hospital tank spun up and ready to go. Transfer all fish to it (because you have to assume if one has it they all have it) and start meds right away.
And yes, QT all
new inverts and coral in a similar manner in a separate QT tank from the fish. (The ones you already have in there have done any damage they're gonna do). Corals can have pests on them that can wipe out your existing DT corals. And both inverts and corals can carry fish parasites which, once introduced to your DT, can wipe out your fish. It's very possible your next outbreak will come from throwing in a "harmless" hermit crab or coral frag from Petco.
Required reading—
https://www.reef2reef.com/threads/how-to-quarantine.189815/https://www.reef2reef.com/threads/my-current-qt-process.483371/https://www.reef2reef.com/threads/coral-invert-quarantine-time-frames.334584/https://www.reef2reef.com/ams/quarantine-setup-made-easy.755/https://www.reef2reef.com/threads/ammonia-control-in-a-hospital-tank.296119/https://www.reef2reef.com/threads/how-to-acclimate.192300/https://www.reef2reef.com/threads/medications-to-keep-on-hand.213574/https://www.reef2reef.com/threads/ratios-for-dosing-copper-power.385871/page-3#post-5336893