advice please

[Northern monkey]

ive got a 70ltr eco style tank which I want to use as a marine tank, I know it's not biggest of tank but I'm dedicated by the space I have.
now I'm going to be getting the water pre made at the lfs and only looking at keeping soft corals as the tank comes with led.
im looking at between 4-7kg of live rock would anyone recommend what' amount would be best?
Adding live sand again roughly how much would you recommend?
i have a tunze 6025 which maybe overkill on the ph side of things but I'll see what happens.
and last thing is the the filteration as it comes with a pump that sucks up then goes to a spray bar that drop water over 2 plastic grates which have small amount of carbon and a thin white filter stuck to the other side, would should I say dump the whole system for a external filter or keep it and just replace the factory stuff with a bag of carbon, rowaphos and filter wool?
sorry for all the questions thanks in advance

chris

[shesacharmer]

I would NOT use any kind of a spray bar with salt water or you will have salt creep EVERYWHERE!

[tzylak]

Hmm 70l is quite small. = challenging to keep it stable
Using 1 lb/gal rule you'd need 9 kg of LR. -That rule is open to interpretation.
9kg of sand would also be a good number to start with.
IMHO HOB PSkimmer is better than any filtration system. It cleans and aerates.
I am only 4 months into the hobby so take this with a grain of salt.
Good luck.