My idea is to use tanking slurry I have done this on a few small ponds, this one being mine seems a good place to test it. Worst case if it fails I can drain it and fibre glass.
What unusual looking fish! Don't they need water lolStupid warm weather made work busy so not done much, poor fish stuck in a pond that's too small. Still got some done.
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That's what you think lolwater would help keep them out