C. albida red leaves

I'd say either the Seachem Flourish Root Tabs or the Jobe's Houseplant Food Spikes. In GWAPA we've used both.

Lately, I've gotten away from using urea based fertilizers because they tend to raise the pH to damaging levels.

"When a urea particle dissolves, the area around it becomes a zone of high pH and ammonia concentration. This zone can be quite toxic for a few hours. Seed and seedling roots within this zone can be killed by the free ammonia that has formed. Fortunately, this toxic zone becomes neutralized in most soils as the ammonia converts to ammonium. Usually it's just a few days before plants can effectively use the nitrogen." - Curtis J. Overdahl, George W. Rehm and Harvey L. Meredith; University of Minnesota.

http://www.extension.umn.edu/distribution/cropsystems/DC0636.html

Lately, I've been using MSU, Dyna-Gro, or Better-Gro fertilizers which are urea free for my Crypts as well as my orchids.

Cheers.
Jim