I use all kinds of lines. Yes, I primarily use fluorocarbon level lines and most of my lines are clear however, some of my favorite tenkara lines are tapered nylon. Other great choices are furled lines and the first line I used to learn tenkara was a lite fly line cut to length. At one time or another, I've used just about all the line type available. As I develop further, the different types of tenkara that I do (keiryu and honryu tenkara) I tend to choose a couple of different line types for specific types of streams and rivers. Yes, primarily I use a #3.5 line, if I had to choose one, that would be it but over the years, I have developed a systematic approach toward crafting lines for my type of technique.
Sunline, Fujino and Nissin make the lines I use. Seaguar makes the conventional tackle fluorocarbon line that I create my favorite lines from. Seaguar InvizX in 15lb is the equivalent size of a #3.5 tenkara line. I cut a piece to length and add in a stepped downsize of fluorocarbon and then a tippet ring to terminate my line. This system helps presentation by making a lighter end line that transfers the energy easier as it loses energy. It also helps with accuracy and is a smaller presentation to the target fish I am going for. Thats the basic concept of my favorite line. I might use a Nissin Oni in PINK with a clear fluorocarbon tip or a different type of fluorocarbon main line suggested to me such as Seaguar Tatsu by a suggestion from Dr. Worthing.
In the past, I might have made a line before I went fishing but that is not the case now. I have a trio of rods that I use for ALL of my tenkara. Rod lengths from 3m to 5m divided by 50 cm lengths with overlap. With my line storage system, I can easily travel and be prepared for all of my tenkara.
I use a wooden spool in the bag I carry. I keep the spool in my bag to stow the line when I am hiking and or moving with the rod nested. I also use it for any rod that I am using at the time. The line storage system allows me to choose a type and length of line, my wooden spool lets me store it as I move to and from the stream.
This is the totality of my tenkara system. I do the homework, put it all together and take it on the road. It is much better than figuring it out each time I go fishing. I choose a rod first, line length second and away I go.
"Tenkara is a game and you play the game by the rules you decide. Eiji Yamakawa"
If I travel with a couple of rods, I take the line storage system with me to choose lines from. Again, the wooden spool is what I use for the days fishing.
In my fishing bag, I have a spare line stored on a card spool as a backup. Again this is a pre made line in the configuration above longer than any line I would use on stream. I choose a backup that long in case I am using a 4.5m rod or a 2.4m rod. I can cut this backup line to the length needed. All of my tenkara rods are based on the ability to cast a #3.5 level line so that back up line can be used for any of my tenkara rods.
The line storage boxes I use are inexpensive and available from general online retailers. I did not purchase them from a tenkara specific store. I use a hair tie to hold the lines on the spools and a black marker to designate the line type.
If you are still reading, below is a list of other line articles that I have written that might be interest for you.