filtration / live sand

Question
Hi there, first of all live sand ,can i add this to my tank now even though its been up and running for 4mths ?
i have a 40 gallon tank 20 kilo live rock and protein skimmer.
FISH  ,5x cromus ,1x wrass,2x damsels,2x dancing shrimps...
2ND- FILTRATION whats right and whats wrong,i was told that live rock alone would be all i needed for filtration .now I've herd people say i would have better filtration if i added a fluidized bed and canister filter,is this true or are they just wanting my money.
reason i ask is because i told my LFS that i never have success with clown fish and that they die on me after getting ich ?

Answer
to be honest live sand is a bit of a gimic, i cant see anything still being alive in there after weeks on the shelf.

i guess you could add sand now, but beware it will be seriously cloudy for a good 12 hours.

i personaly rely on liverock to do my filtering. the fluidized beds do work and all the canister filters are good for is to house non-conventional media, carbon, phosban and de-nitrator. just make sure you have good water flow through and round the rock work to get adequate filtration.

funny you should say that about clown fish, a friend of mine has the same trouble, we looked at the system to see if there was anything there that could stress them ad came up with nothing....1 of lifes mysteries i guess