Hi Kiwiraka,
Thanks for this advice. I'll take it on board and see how I get on.
I'm not sure of the exact dimensions of the tub, sorry. I'll check it out, as it still has its shop stickers on.
I'm going to leave the plug as a functioning drain, straight into a sump of scoria. Hopefully this will allow me to be able to quickly change the water. However, I imagine that putting hose water straight in to replace the old water will be no good, as it will be chlorinated? I'll get hold of some big plastic barrels to keep the water in until it's gassed off.
I've decided against putting crawlies in. It'll just be eels/an eel.
Thanks again,
Tom