In July of 2007, I started investigating reef aquaria.
Most novice reef tanks are as if a space alien wanted to make a garden. The alien says 'Hmm. These are all pretty plants, and they just need water occasionally, lots of light, and some air circulation.'
So the alien looks for pretty plants, that are reported to be 'very hardy'. And it picks a cactus, a pine tree seedling, some moss for ground cover, a rose bush, and some squirrels.
At first, everything looks great! Wonderful landscaping in the 10' cube, a layer of topsoil, and plenty of light. Toss in a few nuggets of 'Earth Critter Chow' every day for the squirrels, and all is well.
The squirrels are unhappy, being nut feeders. After a week of near starvation, the squirrels finally start choking down the high-protein 'Earth Critter Chow', their metabolisms rebelling against it, and slowly causing kidney damage to the poor things.
A month down the road, the moss is drying out, and the pine tree sapling is starting to wither. 'Hmm' says the alien, I should look on the GalaxyWeb. And sure enough: 'Earth plants like a lot of water'.
So the alien turns up the sprinkler, and sure enough, the moss comes back, and the pine tree is happy.
A month later, the cactus begins to rot, and the rose bush is covered with mildew! And the squirrels are get very fat, but acting sickly. What to do! And the pine tree is already 10 ft tall, and about to outgrow the cube!
The moral of this story is that one organism's optimum conditions may be non-optimal, or even lethal to another. Reseach your critters, get them to the food, water (chemistry and flow!), and light *for their species*, and trade the one you can't support in your tank for the ones you can.
Everyone wants a Garden of Eden, with a little bit of everything colorful, but the real world doesn't work like that. A novice would have much better success tuning his tank for one specific reef geography (Fiji high-reef and Florida's Gulf of Mexico coastline reef seem popular in the states) and getting only organisms from that geography, than to go to the LFS and say 'I want that one - it's pretty!' and do their research when they get home.
Of course, you could always just have an aiptasia & hair algae tank.