I was told by the shopkeeper, where we bought these, that the fish in our pond were eating the roots. Some Googling validated that, so we netted off a section of the pond, and yet they seem like they've gotten worse. Water Ph is a tad high, but ammonia, nitrite, and phosphate levels are all good. Any advice would be greatly appreciated!
Hi, MaybeDave, koi will eat water plant roots if there's not enough organic material suspended in them. The roots are quite filamentous and are great natural filters for everything from algae to dead bugs and in between. Normally koi just "forage" in the roots, sucking out good stuff, but if there's not enough in there, or if they're not getting enough koi food, they will eat the roots themselves and the plant will start to die.
While water lettuce does not flower and is not a particularly attractive plant, it is so useful to ponds in so many ways that the pluses far outweigh the minuses. As I said, it's a great natural filter but it also proliferates rapidly through rhizome-like projections that extend in all directions, each rhizome producing a new plant bud. The plants will stay held together tightly by these umbilical cords and if left alone will eventually cover the entire pond surface. Once a bud's own roots are long enough to serve as ballast, if the rhizome is broken, the new plant will stay floating upright, surviving and flourishing on its own. Here in Florida, summer days provide enough heat to raise pond temperatures above 90F. I hear poached koi does not taste good. By allowing the WL to cover large areas of the pond, the shade provided by the heavy leaves can make a difference of five to ten degrees in surface water temperature. In addition, this plant cover gives fish a way to hide from predators and relax in privacy (they can not only see people next to the pond - they can recognize them!).
Your WL looks quite a bit different than that commonly found in Florida. The leaves appear to be thicker and smaller. Don't worry about the dead outer leaves. They are a result of earlier root damage. If you want to replenish your supply, I recommend you pull out 10-15 of the healthiest looking plants (judging by root condition). You can pull off the outside layer of the brownest looking leaves as long as the plant does not flop over when you put it in a water tank. The idea of "netting" your floating plants to protect them is a logical one, but sometimes there are other, in-pond causes of plant disease such as pH extremes, organic toxins or pathogens that can confuse the situation.
I use ordinary tap water and a plastic planter holder (about 3 gallon capacity). Don't worry about nutrients yet. Put your selected plants in with enough room that you can see how fast the roots are recovering in a week or two. Place the planter in partial sun. After two weeks, you should notice new growth or the appearance of rhizomes. At that point you can put in a pinch of Miracle-Gro blue crystals. If the water is beginning to show algae or dirt don't worry about it. Pictured below is a planter of WL I grew from ten plants I rescued from the koi 3 weeks ago. There are a couple brown spots from the original damage but on the whole, the plants are ready to go back in the pond.
The other photo shows one of my two connected (by stream/waterfall) ponds, each a freeform concrete shape two feet deep and about the same volume as your pond, with the top twelve inches of coaming coated with oyster shells in mortar. I also have alisma plantago (bottom right) and water lilies growing from pots about a foot below the surface. The plants on either side of the waterfall are four o-clocks growing wild.
Good luck with your pond!