Then I added 5 cories and a pair of rainbows by third, etc. Not too many, but enough that you don't go crazy staring at a mostly empty tank. After the tank has been up and running for about 8 weeks, you can start thinking about adding fertilizers to the water. Measure your nitrate level, to determine if KNO3 should be added. Also figure out if you should add a trace mix.
Iron deficiency shows up quickly in some plants.
New leaves will be yellow or pale. My favorite iron indicator plant is Frogbit. It will show an iron deficiency within a day or so, and leaves will regain their green color in just a couple days once iron levels are good. Most trace mixes are prepared so that by adding enough to get iron levels right, the rest of the elements are right too. One is from Dave Gomberg's JCF Systems .