The fact that there are no plants in the tank could be a factor in this. I would probably advise planting some, at least that way the algea has somwhere to grow other than on the glass itself
It's been a while since I've had fish, but I used to have this problem in one of my 4ft tanks. Similar setup to what you're describing, but with a 10" Plecostomus, and a 12" Tilapia Maria - It basically ended up being that my pump simply wasn't powerful enough.
Like Kristine says, sunlight & UVB exposure can cause the algea to grow much quicker and thicker, although UVB can not penetrate glass, any reflected UVB can cause them to grow quicker.
I used to have two methods of cleaning the tank when there were algea problems. One was simply a razor blade on the end of the cane, and I'd just scrape it away. The other was a super-strong magnet. Two pieces - one inside the tank, one outside. The outside piece had a velvet type covering on it so it didn't scratch the glass. The inside piece was like the plastic part of velcro, enough to scratch off the algea, but not to do damage to the glass.
I also had another tank, which was the same size, that was basically a community tank full of guppies, mollies, platies, etc. and we never had algea probelms in that tank, although it was on a much larger pump & filter.
Like Kristine says, a huge plecostomus is a nice attraction to a tank, but they do get kinda lazy when they're real big, so you might want to think about getting some smaller ones (remember, the smaller ones will usually need to eat a lot more, so they can grow into big ones). You might also want to think about some
Kuhli Loaches. They stay real small (about 3" total length), are super fast so can evade many larger predators in the tank, and can squeeze themselves into extremely small spaces for extra protection. They're another great fish for preventing buildups of uneaten food & algea at ground level, although they usually won't eat it off the sides of the glass. At least if you've got Kuhli's in there keeping the ground cleaner, you know your Pleco's are gonna be up the walls
