Sorry to hear about your fish loss but this does now give you an ideal opera unity to set up the new tank and do a fishless cycle.
Transfer anything you can from the old tank to the new, bacteria is on all surfaces from gravel to plants and rocks but the majority of the working bacteria is in the filter. Transferring either the filter or just the media into a new filter will kick start your cycle.
Dose the tank to 4ppm ammonia and check the ammonia, nitrites and nitrates daily. You should find that the ammonia falls quite quickly followed by the nitrites. Don't worry unduly about the nitrates getting higher. You may need to redose the ammonia every other day or so to keep it cycling. When 4ppm ammonia is converted into zero ammonia, zero nitrites and loads of nitrates in 24 hours then you are cycled. Before adding fish do a massive water change with dechlorinated water to drop the nitrates to below 20ppm, preferably below 10ppm. If you're not adding fish then keep ammonia dosed to about 2ppm to feed the filter and keep it active until you're ready for fish, but again, let the ammonia and nitrites drop to zero and dilute the nitrates before adding the fish.
Your tank will be robustly cycled for a reasonable amount of fish, not fully stocked of course, maybe about a third stocked would be my advice. Run the tank for at least a month to ensure all is well and then slowly add more stock.
Prepping a tank this way should avoid some of the problems that you had with your tetras.
Lastly, blah blah blah, quarantine all new fish in a dedicated smaller quarantine tank for 4 to 6 weeks before adding them to the main tank. I know this is not always possible but it is the best way. It's heartbreaking when the final fish added to your tank bring in a killer decease that wipes out all your stock (speaking from experience here).
I've finished! ?
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