Hopefully this is your filter.
What are your maintenance procedures and frequency for the filter.
Your tank volume is nominal 18 US gallons.
The most common "real" algae removal fish are:
Otocinclus catfish (Otos) and
Siamese Algae Eaters (SAEs).
I am only familiar with Otos from the literature and posts in this forum. http://www.planetcatfish.com/catelog...pecies_id=1350
as an example
The literature indicates that they should be kept in a group the minimum number of which is six.
This would be too many for your tank.
I am partial to SAEs (as I have 7). http://www.thekrib.com/Fish/Algae-Eaters/
I believe that they should be kept in a group the minimum number of which is three.
IMHO unless you enjoy the appearance of the SAE I would not purchase them but would control the algae growth by employment of the methods set forth in the links in my previous post.
With respect to your neon tetra question I also have had "no joy" with neons or with cardinals and have given up on them (probably been through $30 worth the last year and only 6 remain).
I do have some tetras but mine are too large for your tank.
Have you considered planting your tank and purchasing platies (a rainbow of colors are available and I have found them to be much more "hardy" than guppies).