With all due respect; how do you really know it's the tank?
While I won't say it's impossible (because almost nothing is), it is extremely unlikely, especially if you are using a tank designed for fish; and even if it's not, it's still highly unlikely.
Because certain symptoms can mirror so many other things, sometimes it is hard to say what just ends up going wrong. And without some sort of test to find some of these chemicals in your tank water, there's no way to either rule that out or confirm it.
If many seemingly healthy fish tend to start dying one by one close in time frames, it could indeed be disease; as I would hazard a guess that most of us aren't sterilizing all items in between tank maintenances, and even if you are using dedicated paraphernalia, if you're not hand washing in between maintenances diseases can be transferred that way too (some of them).
There's also water quality; SOURCE water quality. For tap water; as long as it remains within EPA guidelines, you might not necessarily be informed of any changes in it. But this doesn't mean the chemistry can't change overnight, as sometimes water companies might need to adjust the chemicals they add to it to make it safe for us to drink. There are many environmental factors that can occur to change our pH, chloramine content; and ammonia and nitrate might be in our taps too (I have nitrate in mine), and these levels can change.
There are quite a few different types of plastic. An interesting read: http://www.qualitymarine.com/News/Fe...%2810/11/10%29
As littlebitty has mentioned, if a container has been "used" before you got it, there is no way to know what chemicals were in it, and in that case, the tank might very well be poisoning fish, because leftover chemical residues can very easily kill our fish. But the plastic itself? I don't think this is what is happening here.
When I lost my 5 bettas in succession a year ago, they all had the same symptoms, and degraded exponentially overnight. There were no physical symptoms at all; they were all behavior. They were all in plastic tanks, but they were different types of plastic; some were acrylic minibows and some were plastic critter keepers, both of which contain very different chemicals in the plastic. But I have gone on to use one of the very same minibow tanks with no issues.
So to sum up, I'd suggest to start looking for other reasons you might end up seeing the behavior you do, starting with contacting your local water company and ask for recent and past reports on quality, or if you have a well; get someone to test your water. If disease can be ruled out (and sometimes it can't), water chemistry is the place I would look next.