2 x 250 is going to cost you a bit less than 1 x 500
The 250 will perish after a year where it is exposed to sun. I have done it and will never do it again.
When you get a hole in the first layer, it will still hold the water, but it creates fantastic breeding ground for bad bacteria between the first and second layer due to the unaerated stagnant water. Dangerous!!
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; [You must be registered and logged in to see this link.]Koi are not my whole life but make my life whole