65 gal wetdry for a 29 gal?

[vancetish]

would this work... using a 75 gal wet/dry filter with overflow box for a 29 gal sw tank with live rock and minimal fish? setting up the right ratio of water out to water pumped in is a pain. any advice on this.... its currently set up. and running ok with just water and salt.
also, i bought some slate and shale, to put in for deco. is this ok, lfs said it was fine. but i am paranoid that it will melt.

[mygreengoldfish]

I'm not so sure of the slate, if it the same kind I have on my kitchen floor then it wouldn't be a good idea because it flakes apart if water is left on it too long.