plastic water tank flexing

Should a plastic water tank be reinforced to stop it flexing when the ballcock goes up and down?
I've been replacing the rusty metal tanks with plastic ones, and thinking that every time the ballcock goes up and down it flexes the plastic, which could break after a few years.
The one I put in a few days ago overflowed last night.
I could move the holes lower. any advice please?
[g]
http://www.wiki.diyfaq.org.uk/index.php?title=Plumbing_Articles doesnt help with cold water plumbing
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george (dicegeorge) coughed up some electrons that declared:

There's a metal plate with a hole in, about 2-3" square that you can buy (or make) to mount the ballcock through which helps alleviate this.
I wouldn't mount a ballcock on a polythene tank without one.
GRP tanks are stronger but even so, I'd still use a plate.
Cheers
Tim
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On Thu, 18 Jun 2009 11:32:59 +0100 someone who may be "george

Yes, normally.
However, if the inlet pipe is adequately supported then the tank will not flex. Adequately supported includes a fixing of the pipe within a few cm of the ballcock. This can be done if there is something solid like a brick wall very near to the tank, which is unlikely in most cases.
--
David Hansen, Edinburgh
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