Bath tub

We moved into an old house. It is more than 50 years old now. My 1st time to experience dealing with this kind of bath tub heating element where water from the tub goes into the U-turn style pipe and comes right back out into the tub after being warmed up.

Because the house is very old, I expected filth to come out of the pipes. So I bought the cleaner from the store specifically designed for cleaning this kind of pipe. But after using the cleaner, I notice lots of filth still comes out. I thought that once I got it cleaned up with the cleaner, it would be at least a couple of months before a build-up of filth started to come back out again. Each time when I drain the tub after taking a bath, I do try to swish water into the pipes to dislodge any dirt from the pipes before refilling it with fresh bath water. So I am not sure if I had too high expectations. Maybe I need to use the pipe cleaner every week instead of every couple months or so?

Just for comparison, does anyone have a newer house with the same kind of bath tub heating element, and does a lot of filth come out such that it would need weekly or biweekly cleanings?

Is the water in Japan hard water? I am in Kansai. I don't have experience with hard water, but I have heard about it and am trying to learn about it. I am wondering if that is the cause of the build-up of filth that is coming out of the pipes.

Did you clean it more than once?

If it wasn't used in a while it could have lots of crap in the pipe. And take a few goes for it to finally be all gone.

if you want to clean up completely, hire a professional.
or
repeat washing processes many times, because you are non-professional.

if you are still uncomfortable, replace the equipment. it is rather cheap, maybe less than 100000 yen including the replacement fee.
https://www.monotaro.com/g/00346049/

We actually did change the old heating element (like the one shown on your link) and installed a new one already. So I am guessing that it is the actual U-turn pipe that is collecting the filth. I still want to see if I can handle the problem first rather than hiring a professional. Maybe I will ask someone at the home improvement store if there is some kind of a special brush to scrub clean the pipe.

https://www.youtube.com/watch?v=Fxdm8hq9zo0
https://www.youtube.com/watch?v=uk0U-Nmz4TU&t=128s
じゃぶじゃぶクリーン
try this one.. reheating pipe cleaner, amazon for 6430yen, for 2 times use.

because you have a new equipment, you have a long time interval to encounter the same problem, especially if you use it cleanly.
the debris are mostly mold grown by feeding on human body skin.
why do you want to make scratches to a new clean heat exchanger ? scratches will become anchors to catch debris.

Long story short, I'm not surprised at all.

In any case, 50 years is extremely old in Japanese standards. Perhaps you were introduced to this house through an agency that focuses on preserving old houses. If so, you should ask that agent. They would be more experienced in dealing with old bath tubs. Asking and getting an estimate is usually free.

But bath tubs in Japan are supposed to age and be renewed in a couple of decades or so. I would just renew the whole water supply tubes in that house.