You need to keep redosing ammonia until both ammonia and nitrite return to 0, 24 hours after dosing.

You could try adding Epsom salt to the tank at 1 tablespoon of Espom salt per 5 gallons. Keep it at that level for a couple of weeks by redosing if you do any water changes. Note: Don't do this if you also have soft water fish in the tank, instead treat the sick fish in a separate tank.

You can put it away except for the filter - run that in the big tank alongside the main filter. That way both will grow some bacteria and you can move it across to the small tank for whatever fish you have in there at any time. Note that when you do that you'll need to monitor both tanks for ammonia nitrite as each filter will have only part of the total bacteria needed and will need to "catch up" to the stock level.

There's almost always nitrate in the tap water to start with, so reading from your tap will give you a baseline to work from. Fully cycling takes a few weeks.

Yes. At the moment neither tank is cycled so ammonia from fish waste will start to build up in the water. In the larger tank, there will be a smaller build-up because of increased dilution.

Fishless cycling is where we use a substitute for fish waste (household ammonia) to grow the beneficial bacteria that eat ammonia and nitrite. Neither tank has these bacteria in any number so you may as well cycle the big tank with the fish as the smaller one. Do a partial water change to remove any loose fish food etc and move the fish to the bigger tank then test daily and water change if ammonia and/or nitrite rise. This guide to dealing with a fish-in cycle will help: https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm

Test daily and water change if you see ammonia or nitrite. I would seriously consider putting the fish in the new tank. They are going through a fish-in cycle irrespective of which tank they are in, but in the larger one there will be a greater volume of water, so toxins will be more diluted and thus less of a problem than the small tank.

I would say so, yes. Well done......your fish will thank you for it and you'll have much more choice with a larger tank like that.

No, they aren't dead, just have been lacking some nutrients. Note though that they should be attached to a rock or piece of wood, not planted in the gravel as their roots will rot if in the substrate.

Yes, it's the danios that need the length so if you return them, an 80 litre is a good choice for a small community.

It's worth checking eBay and other sites for second-hand tanks. Often they come with a filter, heater, etc, and will be hugely less expensive than new. Avoid any that come with fish though as you may be buying someone else's mistake!

I'd suggest if a bigger tank is bought that the fish go straight in. They'll be cycling fish-in either way, but the bigger tank would dilute the toxins more and be a safer option than leaving them in the small one.