What parasites are you treating for?
The dose I always use for skin flukes and chilodonella is 15 -25g per litre. The fish can be placed in this for up to 15 minutes, but as with any short term 'dip' treatment, remove the fish if it shows signs of stress.
At 25g per litre, skin flukes will literally explode within a couple of minutes or less.
I have seen platies thrown into marine tanks (to feed marine lionfish) survive for more than a day! They may have lasted longer without the lionfish there!!
Use a litre of tank water and just put an airstone in maybe, although for the time period, you should be fine