If you want clowns you will need a bigger tank (I think). You will also (most likely) want to put something in the tank that will host the clowns. If that means you want an anemone, keep in mind that you are going to need TONS of light (fluorescent tubes won't produce enough, no matter how many you get). Its easier than it used to be to light a nem, there are PAR 38 12w and 15w bulbs that produce obscene amounts of light. Metal halide lamps work too, but they make a ton of heat, so a chiller might be required and take about as much electricity as a refrigerator.
I know hammer cora, glaxia, Goniopora and other LPS and softies will host clowns, but I wouldn't keep these corals in such a small tank. I have seen Galaxia put out sweepers as long as my arm (I'm 2 meters tall), hammers have long sweepers too but not like galaxia and Goniopora melts like butter and feeds heavily from the water column (hard to achieve with a skimmer but skimming is almost required in a small tank with fish).
These rules of thumb people talk about like water change schedule and WPG are to be taken with a bag of salt. Many people argue that you should change water as infrequently as possible for stability's sake. Never ever ever ever ever follow "x number of species per gallon of water" rule. If you put 1 hermit per 1 gallon of water, 2/3 to 3/4 of them would die and be eaten, not to mention they would eat all the worms and 'good guys' out of your sand and rock and then eat each other. I do weekly 15% changes, I have no mechanical filtration, no skimmer and no sump I only do frequent WC so my tank looks nice, but I could skip a month with no ill effects. I also have 17kilos of rock in 109 liters of tank.
To answer your question of what to put back there, I would only put carbon, and change it 2x a week. For the first month you can put mechanical media to collect the die off from the rock but running it continuously will just cause your nitrates to shoot up. Leaving the debris in the tank will allow animals to process it and for it to seed your sand bed.
Based on what you have said, I can safely recommend some basic things for your setup:
- A deep sand bed - 10cm of sugar grain sand
- A sump - One of my biggest regrets, really wish I would have listened to everyone on this one. There is only so much you can stick on the rim of your tank, and it all looks like crap. You can fill your sump with your DSB(deep sand bed) and your favorite method of export (your vegetative growth). Drill that tank, you wont regret it.
- An open top/lidless tank - Gas exchange is important and with an open top you can put adequate lighting over your favorite invert.
- Par 38 LEDs - These things are CHEEP (check ebay) and put out piles of light with almost no heat or electricity consumption. You can totally keep nems under these.
All my advice comes from the idea I abide by, more environments, the more diversity of life, and the more diversity the healthier your ecosystem will be. Although, there is more than one way to skin a cat.
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