Keiyu anglers use a much thinner line than tenkara anglers, thin enough that even if it was hi-vis, you wouldn't be able to see it. To indicate strikes, they use "markers" that are placed on the line above the water's surface. Since the markers are not intended to suspend the fly, they do not have to be nearly as large as the indicators western fly fishermen use. They only have to be large enough to see. Also, because they are in the air rather than floating on the water's surface, there is no hinging effect at the indicator. This gives the angler a straight line from the rod tip to the hook for maximum sensitivity.
There are two main types of markers. One type is just a knot of polypro yarn around the line. The other is a little rigid plastic flag that is threaded onto the line. I have tried both and much prefer the yarn markers. A knot of yarn weighs much less than the plastic marker, which is just heavy enough to contribute to line sag.
markers can be adjusted easily. I try to fish just off the bottom, but the bottom is almost never smooth. If you never feel your split shot (or bead head fly) bump a rock then you aren't deep enough. If it bumps frequently, it will probably get snagged. Moving the markers is so easy that it is not a problem to adjust your depth frequently.
Because keiryu markers are above the water's surface, and your entire line is essentially tippet material, you are much less likely to spook even highly pressured fish.
The Daiwa "Bright" markers come as 20 meter spools. Since you only use a few inches at a time, a spool should last well over a year. This is by far the most cost effective solution for Japanese keiryu markers.
On the back of the spool there is a slit to catch the line. One revolution (circumference?) of the spool provides sufficient material to make three markers (the minimum number required to show whether your line is straight or slack). With a straight line, you will feel many of the strikes. With a slack line, you won't. To get three markers, though, you have to be very careful with your knots. I now tie them so I get two markers per revolution, and use yarn of two different colors. This gives four markers, two of each color. I alternate them so my markers are yellow, orange, yellow, orange, but do it whichever way works best for you.
On the back of the Bright Marker package is an illustration showing how to tie the markers onto the line. However, the illustration to the left, taken from the Daiwa website, is easier to tie and is just as effective in creating a knot that you can move but will stay where you put it. Cut the tag ends to about 1/4" or a bit less. If that makes them too small for you to see easily, leave them a bit longer the next time. Shorter tag ends will be less susceptible to being blown around by a breeze.
Daiwa "Bright" Markers - $4.50
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The Owner Pro Markers are a bit more convenient than the Bright Markers. The package contains fifty pre-cut markers - 10 each in fluorescent pink, yellow, white, orange, and chartreuse.
Each pre-cut marker is about 3" long and is just knotted around the
line. The illustration on the back of the package shows just a simple
overhand knot, although I would suggest using the knot illustrated above with the Bright Markers. You will still be able to move the marker when you want to, but it will be a bit less likely to move by itself.
After you tie the marker around the line, the standard practice is to cut the tag ends
relatively short. They don't need to be longer than about 1/4 to
1/2 inch to be visible. If they longer they will cause more wind
resistance, which will require a heavier weight
to be able to make the cast and your line will be more affected by any breeze.
Orange, pink and chartreuse markers are extremely visible, particularly if you use more than one color at a time (upper right in photo)
Because the markers are pre-cut they are very convenient to use. Perhaps the biggest advantage the Pro Markers have over the Bright Markers, though, is that you can very easily use markers of different colors. All the colors are quite visible, but using more than one color at a time really makes them stand out against any background.
Both illustrations on the page show three markers about 8" apart. I now use four (two each of two different colors, alternating), with the markers only about 2-3" inches apart. If they are far apart it is easier to lose them in the background.Owner Pro Markers - $5.00
For fishing with weighted nymphs or split shot, where you want your nymph or shot bouncing along the bottom, the yarn indicators are nicer than using a hi-vis line because the depth from fly to marker can be changed in an instant, just sliding the markers up or down the line. The knot shown with the Bright Markers makes them easy to move, but they will stay put after you move them. (If you find they still slide, take three turns through the loop rather than two.)
A hi-vis line works nicely if you are fishing an unweighted worm in shallow water, but if you are using weight and fishing deep, the markers are more sensitive and easier to adjust for depth.
Also, in low light conditions, the markers are really much more visible than any tenkara line.