starting a nano, what do I need to know?

[i have crabs]

i have a 29g biocube i have a bunch of different corals in it and 1 very small bubbletip anemone wich are all doing great under the stock lighting i also am looking at getting a few different shrinp for it and i expect no problems, i would stay far away from the horseshoe crab though, its a bad idea.i have a seio 820 in mine but its a tad strong so i think a 620 would be perfect. its a very easy tank to care for you just have to watch the water level so it dosent start sucking air in the return pump i replaced the filter cartridge with a sponge, after a few days it was restricting the water flow.other than that everything is ok by me