What's been eating my Alisma plantago?

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In my four years of ponding in central Florida, I've utilized several types of plants. Each has its unique strengths and weaknesses. I've been using submerged potted Alisma plantago for its stately, high-reaching leaf stalks and pretty white flowers - as a contrast to water lettuce and water lilies, both of which offer no real profile above the pond surface (except when the lilies blossom).

Up until this year, my Alismas have exhibited no problems. When leaves would die, their stalks would fall away and fresh stalks take their place. There did not seem to be any diseases or insects that would plague them. However, this year something has been eating my Alisma leaves. I can only assume it is an insect or larva of one but I have never encountered any creatures on them. As the photograph indicates, the leaves tend to be eaten right up to the veins, which are left intact. Eventually, the leaf looks like a skeleton with only the vein structure left behind.

Any ideas as to what could be eating them, and if so, how to treat them?
 

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Haro, I'd love to have a drop in pH as my tests have been showing strong alkalinity, around 8.5. I suspect this is due to a concrete repair of the upper waterfall as well as the addition of several concrete half-blocks to support planters. Apparently, lime leaches off any unsealed concrete and drives pH up. I was actually thinking that the algae did better in alkaline environments but I can't seem to get my pH down despite several treatment attempts using 2 cups of vinegar diluted in a bucket of water before adding it to the waterfall reservoir.
 
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Haro, I'd love to have a drop in pH as my tests have been showing strong alkalinity, around 8.5. I suspect this is due to a concrete repair of the upper waterfall as well as the addition of several concrete half-blocks to support planters. Apparently, lime leaches off any unsealed concrete and drives pH up. I was actually thinking that the algae did better in alkaline environments but I can't seem to get my pH down despite several treatment attempts using 2 cups of vinegar diluted in a bucket of water before adding it to the waterfall reservoir.
if you do try and lower your pH, do so VERY slowly, like over days not hours; fish can handle high pH that is stable but can't sudden pH swings.
 
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Yeah, I get that. Thanks for the warning. I'm thinking that working towards a pH of maybe 7.6 over a couple of weeks would not be noticeable to the fish but would let me know over that time whether algae are diminishing or flourishing with the change. I used two doses of Algae-Dissolve about a week apart followed by Sludge Destroyer and it seems to have reduced the scummy matting cycle but there are still free-floating algae discoloring the water. I really don't like trying to manage water conditions with chemicals so I put a bale of barley straw in each pond today, hoping to get the same great results I did when I first started ponding and had to deal with the initial algae blossom.

I'm also considering the likelihood that the pond purists out there who have said that more than a few fish in a 600 gallon pond is too many - are right. I started this batch of 20 about two years ago when they all were only two inches long (not including two plecos) and 10 small fish in two 600 gallon ponds seemed like no big deal. Well, now I've got five koi that range from six inches to a foot, five more koi that average four inches, 10 comets that average four inches, and the two plecos that are larger than seven inches. My biggest koi must think it's doing the Daytona 500 because it has to turn left all the time.

Anyway, if this algae situation doesn't straighten itself out soon, I may have to downsize the population by rotating out the biggest fish. I was at PetSmart in Daytona Beach today and saw that assorted 2" koi were going for $29.
 

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