Cheap efficient insulation material advice

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Long story short, Im trying to provide my aquarium with the best quality water I can provide. This necessitates having a water butt for aquarium water preparation outside.This will be heated to the same temp as the tank - 25.6 degrees.
Obviously, Id like to try and keep this temp constant and not waste energy keeping it there.

This will be a 250litre butt and is a squarish design. I plan to build a "shed" around it and fully insulate as best I can.

Any advice on whats best to use in terms of £ cost vs effieciency.

Current thoughts are 150mm polystyrene, but Im open to ideas.

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I plan to wrap the butt in foil before hand....

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Muncher, thats about where I at. Ecept Celotex is dear as an 8x4. Im thinking Jab or even straight 50mm polystryene to give 150mm....

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dirkgently said:

To be honest the ?t won`t be that big, 150mm Celotext should be more than ample.

The what Dirk ?

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Good point, ta.

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Mr GrimNasty said:

Don't forget to insulate top and bottom too!

Nope, already in my mind.....

Sit butt on top of insulation and have lid of "shed" insulated to same depth...

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Dr Mike Oxgreen said:

Wouldn't it be cheaper to buy R/O water from your local aquatic shop as and when you need it?

Not really.

I change 250 litres weekly, and RO would also remineralisation - takes time to settle, so needs a butt.