Thanks Dave & Leigh & Barry,

It seems like my problem has been solved. The day I posted my question I
introduced 4 SAE's to the aquarium. This is what happened:

Next day, Tuesday: no change

Wednesday: algae 50% gone

Today, Thursday: algae 90% gone

I did have some Otto's in the tank but even though they look like they eat algae
all the time I don't really see any result. With SAE's result was very fast.
That makes me think if it makes any sense to keep Otto's. Do Ottocats eat any
type of algae SAE's don't?

thanks,
Marvin