would I need to cycle the HT... could I use a bottle of bacteria or is it too late to set one up?
There are many ways to seed a tank and like above stated you can use items from your DT but remember those items will never be able to be used in your DT again.
I prefer TTM but for most copper treatment is easier.
The quickest way to seed you HT is to just use sand 1 rock and water from your DT.
You essentially skip the cycle...now you will just be doing daily water changes and keeping the copper at therapeutic levels. I found a 10-20% daily worked for me but I also had 6 fish in a 20g long and 2 were tangs so I had excess food and poop I wanted out asap. So you might be ok with going longer than a day with a change depending on the how much they eat or poop.
THIS IS SUPER IMPORTANT Remember to add copper to the NEW water you are making so the level of copper NEVER drops.
The alert badges are a good way to go but can be hard to read as most of the time you need to hold a flashlight to the back of the badges to get a good reading. That's why I just changed the water daily. Kept my mind at ease.
This is also why I prefer TTM (Tank transfer method). There is no medication and you dont have to worry about ammonia as you are moving the fish to a new tank every 3 days. Ammonia has no time to build up. And ich has no time to reattached to the fish. It's a win win but is more time consuming and people think it is harder.
I have used BOTH methods and both work.
I used seacham cupermine and seacham copper test when I did the copper treatment.
I lost 3 fish due to the freshwater dips. If you dont have to do the fresh water dips DONT. It does temporarily alleviate the symptoms but some fish aren't strong enough for it and you have to treat the fresh water just like it tank water. It has to be airated for 30 minutes before the dip and most airate while dipping in just FW. The temp needs to be exactly as well. A lot of fish ACT dead and tangs are the best at acting. Also keep them moving if they stop in the fresh water amd no more than 5 minutes, usually 3-5 is the time that takes the nasties to dislodge.
There are a lot of methods and which ever you choose we will be here to add advice and guide.
Good luck!
And yes adding bottle bac is never a bad idea, do it daily if it makes you feel more confident. It won't do anything negative.
It's sad but this wont be your last encounter with ich...the market is somehow flooded with sick fish. I have order so many from the so called large companies and not a single fish has come in 100% healthy.
The best thing to do is get set and ready to QT ever fish you get!