Hello Spottydotty & Minnie ) I can see where your going with this idea as I also considered something similar while back but instead with a kids pool! The main concern with a play tank is the water parameters are atleast similar but definitely treated with good water conditioners that will deal with chlorine nitrate/nitrite and very important metals(they cannot handle even minute traces in water) also it needs to be a similar temperature and preferably water hardness so you can quickly see how much effort would be required in order for them to have more space! My suggestion is go with the 20 gallons tank from the onset! It will be no more work than the smaller tank in fact if cycled correctly and good housekeeping schedule will be far less work# however if you persist with your own idea ensure you get a shrimp net for transporting them as the mesh will not tangle there legs # I don't think it will cause them stress providing like I mentioned you have the parameters of water correct in play tank! If your using this opportunity to clean the small tank never do to much to quickly resist total strip downs# max change 1/3 of water each week clean inside tank ornaments filters substrate using old water you removed# replenish the water from the play tank to the small tank when ready #