Does Eheim Classic Canister filter has "flow adjustment"?

There is no flow adjustment on the Classic series Eheims. You can modify the spray-bar return by drilling more holes or making the existing holes larger (same water volume, but less current produced), or you can use a return line ball valve to reduce the flow (do not restrict the intake line). Or you can just play with positioning of the spray bar - aiming the outlet openings along the bar back toward the nearest glass and slightly downward damps the current a lot.

I have several tanks where I bought an extra spraybar or two, and daisy-chained them together to make one 4 foot spraybar. It really disperses the current, and you still get max throughput from the filter.

Jim