My take on barley.

Prefilter has nothing to do with algae.

Bio filter has very little to do with algae. It just converts ammonia into nitrate. But since algae consumes ammonia directly the bio filter really doesn't do anything algae wise. Green water algae is first in line at the ammonia dinner table so a bio filter in a green pond isn't getting any, or hardly any ammonia to convert. A green pond with no bio filter will almost always have zero ammonia.

So the concept of doing anything nutrient wise to control algae is a very old and very popular myth that will never go away but has no basis in reality.

These days a pretty simple solution is a 24/7 drip into the pond to do a continuous water change. Weekly changes aren't as effective and hard on fish.

UV filter is of course 100% effect when installed correctly.

There are chemical solutions which vary in their effect on fish and effectiveness.

The simplest imo is a small trickle tower or stream. A trickle tower is a stream, just a bit more compact and can be easier to build. Just a pile of rocks with water flowing over it...like a stream. Search for "strawberry pot pond trickle tower" which is one version nice for small ponds.

The reason these (trickle towers and streams) often, but not always, work is that string algae grows on them. The different kinds of algae wage chemical and environmental warfare with each other to stop the other from growing. Green water algae is normally first to establish, produces a chemical to kill string algae as it sprouts in combination with blocking sunlight so string algae has a hard time growing. The TT or stream gives the string algae a place closer to sunlight to grow and that normally gives it enough to grow a bit and then it creates a very powerful chemical that kills green water algae. It takes very little healthy string algae to clear a pond.

Adding plants often gets credit for clearing a pond. Many people add a potted plant and the pond clears in a few days or a week. They then declare to all that the plant sucked all the nutrients out of the water and the green water starved to death...absurd on several levels. A couple of weeks later they start posting..."How do I get rid of string algae?" and never make the connection.

String algae sores do blow into ponds on the wind, but buying a pond plant is almost certain to have string algae tagging along, whether visible or not. Plus the pot gives string algae a higher place closer to sunlight to grow.

Once the pond goes clear you really want to be testing ammonia...risk depends on the fish load, water temp and pH but worth testing to be sure. Although you have a bio filter it's unlikely to be very established so now that it might be getting ammonia so it's like a new startup. But in most water gardens the little bit of ammonia produce is still consumed by algae. And of course if you added a TT or stream that's going to be about 10 times (measured with real tests) better than a submerged media type filter which I assume you have.

Interestingly, if you use a UV filter you can normally turn it off and never run it again after a few weeks of being clear. Why? Cause once the green water algae was killed the string algae inhibiting chemical disappears and sunlight gets to more surfaces and string algae (actually most macro algae) grow and produce the chemical so green water algae can never establish.

With clear water you will notice a green kind of dust on the pond of the pond. You can scoop that out with a minnow net from the pet store. But it will keep appearing. The green dust is green algae algae that has grown and started to die, clumps together and falls to the bottom. So even though your water is clear the green water algae still continues to grow but doesn't live long enough to accumulate to make water green.

These are living systems so there is variation. But that's pretty much how it works. Not that 99% of people will ever believe it.

You are on the right track. And plants absolutely do keep algae away. They use up the nutrients and also shade the pond which also keeps sun from algae also. Floating plants are amazing for filtering and eating nutrients.

As mentioned give the filter time to grow the good bacteria. And some small weekly water changes.

But also your plants need lots of sun to thrive. At least the ones you are using. Water hyacinth loves sun. So maybe it's to much shade for it. And make sure the water flows through the roots of floating plants. When the hyacinth starts multiplying like crazy you'll start seeing clear water. But if it doesn't then you may need more shade tolerant plants. If you can get a little more sun on just the plants and they grow more you'll be in good shape. You just don't have enough yet. Two small plants is not enough.

So try to do those things and wait it out and I'm telling you it will clear eventually. The other week I rearranged some plants so the water flowed through the plants and within a week my water was clear. But I already also had a bio filter that was established. Just needed plants to eat the nitrates.

Why would removing nitrates...assuming higher plants could do that, (they can't actually remove all)...stop algae? Algae consume ammonia before it gets a chance to be converted to nitrate. If you want to remove 100% of ammonia and nitrate as fast as they're produced the best plant to do that is....green water algae.

Ammonia is NH3 and nitrate is NO3. The algae is not after ammonia or nitrate it's after nitrogen which is the N. To get the nitrogen from nitrate the algae first convert the nitrate into nitrite an then convert the nitrite into ammonium...the exact opposite of what bio filters. That's why algae prefer ammonia by a factor of 50 and nitrite by a factor of 20 over nitrate.

Quick web search will confirm...once you get thru the waves of people spreading the same old misinformation.

The single biggest problem with keeping a pond is poor info. When I was building ponds most contacts were requests to fill in ponds. They'd been told and tried so many myths, repeated over and over, that they thought they were the ones doing something wrong. The only thing people do wrong is shop around for an answer they what to hear.

hi

Interesting that nobody mentioned the fish load?? 9 fish in 25 gallons?? You need to expand the volume a whole bunch . The system is being overwhelmed by the waste load!!. I'd either expand or drasticly reduce the fish load gary

There's only one way to tell if the system is being overload, by testing ammonia. Maybe testing nitrite and nitrate also, but manly ammonia. OP provided no test results, but says the water turns green which is a pretty darn good test for ammonia, it's likely zero. So I see no reason to think the system is overwhelmed. It certainly could be. Posts only say so much.

One cure for high ammonia and nitrite is to let a pond turn green. It's a much better bio filter than any of the man made kinds and can be much faster to cycle.

But yeah, like I mentioned, once a pond goes clear you want to be testing ammonia, because you just lost your only working bio filter and it takes awhile for a man made bio filter to cycle. If you do get high ammonia readings then you have to deal with that and also test for nitrite. If you never get ammonia readings the nitrite testing isn't really needed.

And besides, if the OP takes the advice and starts expanding the pond...what are the chances of these fish surviving that duration and transition? Somewhere in the zero zip code I think. So OP will have gone from having no problem fish wise to greatly increasing the risk of deaths. Thank you very much.

And drastic reduction of the fish load is kind of an automatic feature provided for free by nature. Yeah, would have been better to add fish a little at a time, to learn more first but what's done is done.

I'm kind of interested in the OP saying that the pond turns green "about once a week". Is that just a poor choice of words or is something being done? Like a 100% water change, some chemical being added? Why is the pond clear once a week? Don't know. Also the description that there's a thin green layer but also that it's the floating kind may mean OP is having a hard time describing what's going on algae wise. But that's kind of normal for posts.