uhm, i disagree with your
lfs. I have the grey cansiter on my ten gallon and its a small jungle. I have great growth in all my plants in there and its because of the co2 addition. Now, you don't have to buy that $20 set up. You can just as easily do a
DIY set up for a 10 of 15 gallon for a fraction of the cost. The hagen set up isn't very useful after about 30
gal. I know people on this site have used the hagen for small tanks like 5.5
gal or 10
gal.
as for excel, yes, it can be useful for a 15
gal. but I think for the best success, you have to dose it everyday or every other day, without fail. I used it on my 10 before I got the hagen. It worked to an extent, but I think the hagen does a much better job.
search the site a bit, you'll see lots of folks use hagens or
DIY co2 for small tanks.