Also if it is a front yard and there are any kids around, they will throw untold items/liquids substances into a pond, and with fish I would worry about their security/lives.
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People are giving away free plastic ponds all the time on free sites and CL. Especially in the winter when they are tired of seeing them around.
Most of those are around ~100-150G so not really a long term solution for Comets.
Other options are keeping your eyes open for building materials like RR timbers or pressure treated landscape 4x4s. You may be able to make a frame for a liner with some ingenuity and creativity.
Stock pond heater will work well so the pond doesn't freeze over and you may find a deal on a used one but they are pretty cheap to order online.
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I made a reasonably priced pond for growing fish - GF and young Koi way back with a plastic liner pond,
some blocks which were free - salvaged. To add support for the ledges
Pavestone 4 in. x 11.75 in. x 6.75 in. Fieldstone Concrete Retaining Wall Block
https://www.homedepot.com/p/Pavesto...k-144-Pcs-46-5-Face-ft-Pallet-81185/202689750
A new garbage can clearanced out and donated to the Restore due to no lid for about $3.00, to make a trashcan filter.
A small used garden pond pump which I think broke and ended up with the neighbors pump for free when they moved and planned a significant upgrade.
~misc parts and fittings to get the trashcan filter to work right. $15-20 - could have got cheaper at a ReStore. Since I didn't exactly know what I was doing I probably had some errors in choices I made.
Chunked up filter media pad. And other assorted items which could be used for BB
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There are other reasonably priced options like Stock tanks for watering livestock.
There are plastic and galvanized metal
700G round 8'x2'
https://www.ruralking.com/tarter-70...GcZd60el2z7BfYU5Zcwb0Ne6uUxJzYwQaAhu5EALw_wcB
350G plastic round, of course there are long oval, etc.
https://www.amazon.com/dp/B00EE194B...t=&hvlocphy=9028724&hvtargid=pla-349545294362
***JBWeld Marine is great for patching those plastic preformed ponds FYI. Can use it in wet areas.