I thought about pretending this was a shop question (I still might). A few years ago I put in a couple of ponds. Total area about 10'x20'. My goal was a pretty water garden with a rock garden around it.
However before I put one plant into the pond fish showed up... Well now I've grown attached to them and somehow feel worse if a fish dies instead of a plant. Though they ate my water poppy. And I did like that flower.
That said good fish pond maintenance means getting rid of the debris that decomposes in an outdoor pond. If it was a well designed fish pond this would not be a big deal.
However I have a poorly designed fish pond. It has 6" of pea gravel, sometimes called 3/8's minus around here on the bottom. I put flat river rock on top of it. So I bought a trash pump to vacuum the crud off the bottom and of course it gets some of the pea gravel.
I've already had to fix it once. It ain't made for this kinda abuse.
I tried to find an eductor type pump but apparently unless your dredging a shipping lane I'm out a luck.

 

why not make an eductor style pump? It's a pretty simple design, although it would probably launch a piece of gravel a ways. Either that, or find an air powered diaphragm pump. You can find those on ebay, at mcmaster.com, or sometimes if a factory is closing. They are usually good for stuff up to 1.5" around, so should be fine for that.

 

You need a sand filter on the suction of the pump. Should be easy to keep the -3/8" gravel out of the pump. Do a search on sand filter irrigation.

Eductors are pretty good at moving solids, but not very energy-efficient. It would still need a suction filter anyway. You don't want to hoover a fish into it, do you?

 

Thanks! - this gives me an idea or two. Put a reservoir - like an old water heater tank maybe - on the pump intake. The heavier gravel should stay in the tank if I do it right. Just have to figure a way to clean out the sludge. Also might solve another problem. Right now if my scrub brush/suction head plugs up it kills the pump motor if I can't get it unplugged quick enough. The tank would probably buffer that pressure build up for a bit.
Before I bought the trash pump I spent a weekend making an eductor or two. This was after I made a few calls to some pump supply companies around here - one in Bremerton actually. Every one I talked to was helpful (even though I could tell most thought I was nuts) - and pretty patiently explained the drawbacks. Cost being a big one for an eductor big enough to pass the organic debris. Mine worked but it was real inefficient due somewhat to design flaws I suspect.
You know I looked around for a diaphram pump before I bought the trash pump. Couldn't find one that would do the job in and was in my budget. Of course now I look and found two that were cheaper than the trash pump I bought.
Haven't came close to sucking up a fish yet. They tend to stay out of the way. Did get a frog once though.

 

If you are having problems with algae growth, there are some good algaecides out there that are harmless to fish. I use Cutrine plus (Applied Biochemists) in my cooling ponds to keep algae off the intake screens to the pumps. That might cut down the crud in the ponds so you don't have to clean as often.

 

Checked their website and one of the application notes says not for use with ornamental koi and goldfish or for small backyard ponds but it doesn't say why. And lo and behold it's being sold for use in small ponds like mine. I'm thinking it has to do with the copper in a 'closed system' accumulating. I'm not rolling the water volume over like you probably are in the cooling ponds.
I'm checking with some folks because it would be a very inexpensive algaecide for my little pond.
Thanks again.

 

Guess it's always a good idea to read the fine print. Good luck.