Hello. I have a small 20gal tank that I set up as per my LFS. My plan is to keep 1-2 small fish, a shrimp, and some hermit crabs. They provided me with a crushed coral substrate, some fake decorations, and 4 damsels for cycling. I have since discovered this site and now realize I should have used the dead shrimp method and some live rock but I'm 6 weeks into my cycle. My water test levels are: Ammonia 0mg/l, Nitrite 3.3mg/l, Nitrate 110mg/l, ph 7.7, temp 80. Although the ph is low, it appears that my cycle is almost complete and I'm going on vacation for 2 weeks so I'm going to return the damsels to the LFS. I plan on adding some live rock soon. Can I add cured live rock now or should I wait till the nitrites are 0 and I do a water change? I would also like to change my substrate to sand. Can I remove the crushed coral all at once and add the sand or should I change it in small sections as some people have recommended in previous posts. Thanks in advance.