Jenny, some years ago I had five preformed pond shells and a buried fifty gallon plastic drum, linked together with short lengths 3" ABS pipe.
The main pump was in the fifty gallon drum and supplied the water via the filter, to the falls which were sited at the furthest point of the first pre-formed pond in the chain.

The water circulated throughout all five ponds via the 3" pipes all year round. The longest pipe "tunnel" was about three feet in length. And at that point,between two of the ponds,I had a small wooden arched bridge strategically placed, with plants on each side, so it looked like these two ponds were one.
In winter had a small 500 GPH pump that pumped the water around to keep the ponds opened. I also had them protected from the winter winds, with a tent like covering. Plus I had a floating electric, 1500 watt de-icer in place, as a stand-by in case the pump failed.

If I was ever (though very unlikely!) to do this sort of thing again, I would use a much bigger pipe size to make it easier for the larger fish to swim through,4" at least or, 6" diameter, if possible.
But even with only those 3" diameter tunnels, the fish all migrated through those ponds all day long, moving like a conga line, as the sun warmed the water in each pond in turn.
The buried fifty gallon drum gave me a depth of three feet, which was well beyond the frost line here and the preformed ponds ranged from 18" in depth to 24" in depth.
"Horton"

Hi Horton,

Thanks for the reply. How deep was your ABS buried? Also wondering how you connected the ABS to the shells if you don't mind sharing.

I already am planning on using a minimum of 8" pipe between the two parts for fish swimming ease and safety; I'd hate them to get their fins or bodies beat up trying to squeeze through a too tight pipe (providing they do get bigger at some point!)

I have really no choice but to use the pipe boot method that I learned on Greg B's DVD as bulkhead fittings that big are much, much too expensive!

Loved the visual image of your fish conga-ing back and forth all day long! :-)

Jenny, I used toilet bowl floor flanges bolted to the preformed shell, with of course "lottsa an lottsa" roofing sealant parged around the flanges.

I only wish I could have used the pipe boot method to connect the pipes, I would have slept better at night.LOL
With having so many connection points, I was always thinking one of them would spring a leak, but it never happened. When I eventually got around to putting in an EPDM liner pond for myself and pulled out the preformed shells, the connection points were the strongest part of the whole system and took a lot of work to take apart. I had to use a reciprocating saw to cut the ponds apart in the end.

The pipe boot method had been used by roofers, long before pond installers copied the idea. It is a really secure and trustworthy transition connection point.

The 3" diameter pipes I had installed, varied in depth, from about six inches to eighteen inches deep, depending on the depth of the particular ponds I was linking together.
"Horton"

phheeewwwww That's me taking a big breath. I'm petrified of relying on pipe boots to connect the two sections. Something about cutting a huge hole in the liner and then realizing that the boots aren't working. It would be a little late to turn back then!

I'm glad to hear you find boots more secure than flanges (which I consider the bulkhead fitting equivalent for epdm) and lottsa, lottsa sealant which has been my favored method of sealing holes in the liner.

So, while I may still be nervous, I can take a deep breath and try to relax knowing that other people put their faith in pipe boots and that it will all be okay if I proceed carefully.

Thanks "horton" for your feedback!

Jenny

PS Whats the quotes around your name for? I've kinda always wondered.

PS:Maybe "Horton Hears a Who?".
I live in Zone 3 as well but Alberta winters are very long and this year's forcast is for very cold--the mountain ash trees are loaded with berries not a good sign.

I have pipe boot connections in my pond for both my retrofit bottom drain and a new midwater drain that I put in this year to get more flow into my sc. Pipe boots in the settling chamber which is also liner chamber and on discharge as well. Never had a problem.

If you are just intending on using the pipe for the fish to go back and forth it doesn't need to be that deep--at least a foot of water over the pipe to allow for water level changes should be enough. I would go with the largest pipe you can find both for the fish to go through and not get stuck (don't know how big your fish are) and for water flow between the two ponds. You will need to have both ends of the pipe extend into either pond far enough to cap the pipe for the winter. As I use my pipe for the drains I have a fernco fitting on the pond side of the pipe. In the fall I pull the drain and put a threaded cap in the fitting. On the settling chamber side I just use a test cap. The line has a knife valve so I can shut it down--drain the sc down below the fitting and put the cap in on the pond side then shop vac any water out of the pipe between pond and sc (it's only 18" of pipe) and cap the sc side.

In your case depending on how deep your pipe is you can cap one side off then you'll have to drain the other side down far enough to get any water out of your between ponds pipe then cap it. Water left in the pipe will freeze and crack the pipe if you don't drain it.

There is another thread on this forum on overwintering fish under ice in cold weather conditions. I've never had any luck overwintering fish in my pond but like I said I live in -25C weather for three to four months of the winter. Oh joy. Not! Good luck with your project! Patti

Sorry Jenny, I did not see your last question until now. Patti got it right,the name came from reading my grand children's "Horton hears a Who" book.
That is my favorite Dr Seuss story. Those Wickersham brothers were nasty little buggers. LOL
"Horton"

Sorry, this is way off topic, but wouldn't a very appropriate Dr Seuss be "one fish, two fish, red fish, blue fish"? :) I love Dr Seuss!!