Honeythorn
Sugar coating and nicely nicely? I don't think so
Right, the remaining fighting few survivors are as follows:
1 BGK, 5 Guppies, 4 Danios, 11 Neons, 6 Endlers, 1 Pleccy, 5 Roseboras ( I think that's what they are), 2 Apple Snails (if they are still alive), 3 dwarf frogs, 1 Clown Loach.
I have added the air pump/air stone with immediate improvement in fish movements now but with the ill ones it's hard to tell if they are moving on their own power or with current. The water was changed almost completely by my partner (without my knowing) hence the low nitrite/nitrate levels in the hospital tank.
With regards to de-chlorinator, at first yes we did, and with every water change, but have now stopped in main tank.
With regards to tank size and your fish, Your tank is around 16 gallons and will stunt and contribute to the early death of the plec the clown loach and the BGK. The fish store has given you terrible advice with the keeping of those fish. They all grow very large indeed and need tanks of at least 75 gallons .
The kindest thing to do would be to return or re-home these asap. Your tank cannot support them.
You can just about get away with the smaller fish if you do 50% weekly water changes ( or 2 weekly water changes of 25% if that makes it easier )
As for the goldfish, they also grow very large and require at least 55 gallons per pair. Or a pond as common goldfish, shubunkins, comets and other normal bodied goldfish are actually pond fish . Fancies generally grow to the size of a cricket ball and still require large strongly filtered tanks. You were getting astronomical readings in your coldwater tank becuase these are messy fish that produce a LOT of waste. An external filter is really the best thing to use for big fish like these ( more filter media space means more room for good bacteria to break down fish waste )
You should also always use dechlorinator when adding fresh water to your tanks during water changes. Chlorine and Chloranine is very harmful to fish and will not make those who are ill any better.