Puzzled over florescent light behaviour

I've just installed 2 new tubes in my fish tank. They both light up beautifully for about 1 or 2 seconds (no flicker), then go out.
Leaving them for about half a minute or so, I can turn them on again (again only a second or two of light). Mains circuit is just fine as there are other appliances attached and the bulbsworked just fine until I changed them (neither had blown, they were just about 4 years old). Any suggestions?
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