Be aware that with a 2 bulb t5 zoas and mushrooms are gonna be about all you can grow. It you want way more options such as higher light softies and LPS
then you will be much happier with a 4 bulb T5.
To answer whats different about coral compared to fish only. Well for one water quality has to be much cleaner. This means 0 ammonia, 0 nitrite and near zero nitrates, most coral wont handle anything over 20ppm well. Some even more sensitive than that.
Also with reefs calcium, alk
, magnesium and all other trace elements come into play so they need to be at proper levels and stable.
Large or sharp fluctuations also become more serious when corals are involved. Temp, salinity, ph, all must remain fairly stable and in acceptable ranges.
Generally softies will be your easiest to keep corals, with SPS
(Small Polyp Stony) generally being the hardest and LPS
(Large Polyp Stony) falling in the middle.
Softies include xenia, leather, polyps, zoanthids, mushrooms etc.
include acans, frogspawn, brains, trumpets, torches, chalices etc.
include acroporas, montiporas, porcillapora, porites, millieporas etc.
Hope that helps a little bit. Not a small subject and tons of info so read as much as you can on the subject.