It depends on whether or not they are the real thing. I got the "False Siamensis", which looks close to the SAE.
Here's a great link w/ some good info:
Algae Eaters
I put 3 of these imposters into a 38-gallon planted tank, and all they ever did was fight with each other and not eat much algae at all. I left the 2 smaller ones in that 38-gallon tank, and transferred the biggest, rowdiest one (who got quite fat by eating fish flakes) to the 100-gallon tank.
In short, that big False Siamensis didn't do a thing to control algae in my big tank, either. So, I bought a little otocinclus affinis (oto cat), and inside of a month the sides of my big tank, the plumbing, and most of the plants were clean of algae.
I felt like the guy who bought a watchdog to protect the house, only to have the dog just sit and watch the burgulars take everything out of the house! Still, the False Siamensis is a handsome fish and does add some movement to the aquarium.
v/r, N-A