I had a 20gallon rubbermaid container I used as a hospital tank for a gold barb about a year ago. Unfortunately for the fish being treated after 2 months of care and being healed completely then reintroduction to the tank, she was again viciously attacked (tiger barbs). I ended up having her in the "tank" for around 4 months with a heater and a simple Whisper HOB
's work great because they fit over the lip of the tub, you just need to keep the water level high).
What I noticed over that long period of time was the container slowly started to deform. The sides bulged out and while I wasn't concerned the container would suddenly collapse, it did give me pause about using them for longterm care.
If I were to do it again I'd get 2 of the same containers and have them inside of each other to give the walls a bit more strength. But I was dosing with Pimafix and Melafix and (Prime to dechlor the water) and so no signs of the container breaking down.