From my limited experience I'd wouldn't move the fish back & forth. I'd probably just put the filter on the new tank & move all the fish & be done with it. They've been without dechlorinator for so long right? Just make sure you have the time to keep an eye on them and look for any signs of stress. They should be much happier in a tank that's chlorine free and 4 times larger right?
Yeah but there are a few problems:
1) the filter needs an extension cord in that area and dad has to set it up.
2) there will be sand to put in there, which would involve moving all 3 fish back and forth.
3) the tank needs to be moved so that mom can put something under it before I can fill it + I don't have the lid or temp lid yet (hence the 30L in it at this time).
If I'd add the filter, it is pretty tall and the current amount of water will not cover it. If I do add enough to cover it, then add all the fish, Tzuppy will jump out of there like a rocket when there's no lid.
Tomorrow it should be set up properly with sand and a temp lid.