I did a test of this a little while ago. I washed out an old 55 gallon rubbermaid tub and filled it with old tank water, gravel, some sticks/plastic plants... a heater and filter. Put a 5 inch pictus cat in there and two julii cories (I did these fish because they were just extras. plus the pictus is a scaleless fish which would be more suceptible to problems. I don't want to sound cruel but since I had a catfish without a permanent tank I figured it would tell me more)
Anyways it worked for a month or so until I had to use the filter for another tank so I took the cats back out.
From this I determined that at least the rubbermaid brand tubs are nontoxic to fish (probably as I only tested it for a month). Plastic however is made in a number of different ways some of which leach more toxins.
so where did your tub come from? I would test it again with something you don't mind possibly loosing before putting a fish you liked in there though. On the other hand if you are worried and do not want to test you can get some of that pond liner plastic and line the inside. I once did this to grow out platies in kiddie/dog pools that I didn't trust. It worked really well for platies but those would be too shallow for cichlids.
