If you can shine a small flash lite up from underneath the filter, the fluid in it should appear as fairly clear. It might be a little dark amber colored but that is usually OK. If, on the other hand, it is very dark or nearly opaque, it is probably the algae problem. One other consideration about the clear filter replacement. If the tank is fairly full, the line coming from the tank will leak fuel if not quickly covered or clamped off. Not a serious problem, but you need to be ready.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150