Few questions about tank build/move

[ATLL765]

I recently moved and this has required me to move my 20 gallon tank. I figured this was the perfect opportunity to fix all the mistakes I made on the build like not drilling it. I also wanted to completely or at least nearly completely replace the rocks in the display. I have some marco rocks and the cement mix they sell as well, but my question is should I build the scape in the tank or outside the tank? If I build it inside, I have to deal with the space confines of a 20H tank. If I build it outside, I run the risk of dropping the whole structure through the bottom of the tank if a bond gives way or my hand slips or anything like that.

Also, anyone used the e marco cement mix? I must have lost the directions, but I think I recall it only saying to wet the rock with the liquid it comes with then slap on the cement.

Lastly, I figure I'm going to cram as much LR into the sump as possible and since I have a lot of rubble in the tank right now due to it previously being home to a mantis, I figure I should be able to fit a fair amount in there. I figure if I can fit close to 10lbs in there, that should be fine to handle the bioload of a few small fish, right? I currently only have 1 damsel, but was contemplating getting two more since I read they do best in odd numbered groups and I figure it's best to introduce them all at the same time.

 

[ReeferRick]

My suggestion would be to build the rockscape inside the dry tank before adding water or sand. Consider adding egg crate (or plexiglass) on the bottom before building your rockscape to protect the glass if you are not going bare bottom. Next add the sand which may cloud the water but this is a good time to work out the flow you need before everything settles.

Good Luck.