How do you keep the tank and fuel line clean?

Hello,

My question is how do you keep the tank and fuel line clean?
I cleand my tank and fuel line about 6months ago and two days ago suddenly my MK1 stoped on the road, but after a few starting attempts it started again and I was on my way, and this happend at least three times taht day. I was thinking this shoud be the needle valve because I just replaced the carb and it had the original solex needle vale so I replaced it, but it wasn't the problem. So today I went down to a petrol station and bought a new fuel filter and when a pulled the old out I was shocked, it lloked like this:
Maybe I did a mistake because I didn't replaced the fuel filler tubing when I cleaned the system, but can that be so dirty? Or is petrol really so dirty?