I started with a Rio Nano on my 25 gallon tank at first and found the same thing as you. It was very temperamental and required a lot of adjustment. I figured I couldn't expect a whole lot more out of a $35 skimmer. I eventually just bought a
Red Sea Prizm Skimmer. I'm glad I did, it seems to skim better and I don't have to worry about it between cleanings.
Another thing I added to my filter (It's a Fluval 204) a few weeks ago is
Chemi Pure Elite. It seems to have pretty much removed the trace amounts of phosphates and nitrates that I always seemed to have. Like you, I try to do a water change every 7 - 10 days, although I do use RO/DI water as opposed to tap water. Any reason you won't entertain the idea of switching to RO? The filters can be had for a reasonably cheap now, or you could even buy bottled water. That's the way I started. Can you post your water params for us? That might help.
A nice addition to your tank may be a
sand sifting goby of some sort. It could help with your algae problem on the sand as it sifts, and they are really fun to watch "eat" the sand.
