Salis Ali

From what you're describing, I think the pump itself could work, but the bigger issue is keeping it primed. A 1–1.5 meter lift isn't huge, but once you stop pumping, gravity will try to pull the water back down unless you have a check valve or some way to hold the prime.

The 25 mm pipe being full of water shouldn't "block" the pump on its own. What you'll notice instead is that it gets harder to start again if the water drains back into the lower pond. A simple foot valve at the intake can make a big difference because it keeps the line full between pumping sessions.

For moving around 1,000 liters by hand, I'd also make sure the suction side is as short and airtight as possible. Even a tiny air leak can make these homemade pumps perform much worse than expected.

If this is just a temporary solution until you go solar or wind-powered, it's probably worth building and testing. You may find it does the job well enough for occasional transfers, and any tweaks (like a check valve or slightly larger suction pipe) will be much easier after you've seen how it behaves in your setup.

From what you've described, I'd be hesitant to assume the problem is the water itself. The fact that gypsum temporarily clears it, then a heavy rain makes it muddy again, makes me think the pond is getting a fresh load of suspended clay from runoff rather than having a permanent chemistry issue.

The catfish and crayfish probably aren't helping either. Large populations of bottom-feeding fish can keep fine sediment suspended, especially in a clay-bottom pond. Before considering something as drastic as rotenone, I'd try to figure out whether the pond gets muddy immediately after rain even without much fish activity. That can tell you whether the main culprit is watershed runoff or the animals constantly stirring the bottom.

I'd also compare your pond to your neighbor's beyond just the trees. Things like slope, where runoff enters, livestock access, exposed banks, or even a slightly different clay composition can make two ponds only a few hundred feet apart behave completely differently.

As for plants, establishing some native emergent vegetation around the inflow areas can help trap sediment before it reaches open water. They won't fix an existing suspended clay problem overnight, but they can reduce the amount of new sediment entering over time.

Personally, I'd spend a little money on a basic water test (alkalinity, hardness, pH, and turbidity) before investing in more gypsum or restocking fish. A few test results could save a lot of guesswork and point you toward the real cause instead of treating the symptoms.