You need to use a potable water anti-freeze for the least impact. If your marine anti-freeze is intended for on-board potable lines, you are good.

A pipe or hose that's open at both ends (pump or open valve count as open) is very unlikely to break when frozen. Virtually impossible. So I would not add anti freeze or do anything to try and close the pipe.

You can research this but you have to wade thru tons of people who repeat myths, but the info is out there. It's worth learning cause what you're planning is a lot more work and expanse.

The myth is that ice expands. What really happens is water expands when it turns into ice. In homes pipes often freeze where the pipe comes into the house or in an exterior wall. That creates a plug. All the valves inside the house close one end of the pipe and the ice closes the other and. Now as more of the pipe freezes the water expands pushing the water in that closed section. As this continues the water pressure in that closed section increases until the weakest part breaks. Broken pipes from freezing almost always break far a away from the actual frozen section.

When the pipe is open at both ends the water pressure inside the pipe stays at about zero. As water expands into ice it can push water out of the pipe if needed to make room. Problems only happen when there's no place for the water to escape.

This is why opening a tap a bit to drip will keep pipes from busting. The myth in this case is that moving water doesn't freeze which is only slightly true since rivers and waterfalls certainly freeze. In this case an ice plug will stop the tap from dripping but will still allow an escape for water pressure and no busted pipes.

I've seen lots of people have a pond with no freeze problems work really hard and spend lots of money to create freeze problems. Be smarter than the average bear.

Water bug u are always on point and I enjoy reading ur comments. U are probably correct in that maybe last year I had multiple areas frozen which maybe acted as closing off points, which led to the breaking of the pipe. I have approx 25 feet of 2" flexible pipe at varying depths sandwiching behind the pond and a retaining wall. I read directions on the anti-freeze and it claims that its pool/pond safe to all forms of life. I think I would need about 1 gallon. One end leads to skimmer other to my bog. Last yr rain/ snow would enter bog and into pipe. My skimmer side is above water level. I am planning to cover bog this yr to prevent this. I had about a 18" split in hose last yr. I was just going to use antifreeze as extra insurance.

All things are possible.