the % is a rough gauge, for my sandbed, i use the siphon (a open tube at one end connector to a long pipe and the other end is a opening for you to pump manually) --> http://home.pacbell.net/powerfx/images/siphon.jpg <--
i think there are other method to clean the sandbed but thats what i did (to clean up those fish poo and other dirt)
ya the water you replace is either fresh mix salt water or salt water which can be purchase from the LFS.
for now if you dont want to have a skimmer, what i can suggest is you do water changes regularly (if you can't do it, dont bother starting the tank)
Most important thing is that you must let your tank have a good cycling period to cultivate good bacteria, min 4 weeks.
you can buy those phosphate absorber from the LFS.
read up more and you will have a better understanding.
*bros correct me if i am wrong so i can learn too*
happy reefing bro