The sand could cause an issue. I just set up a ten gallon with no cycle using old water and rock from the same system. Im adding sand later. Hope this helps.
No no no the sand is the mother load of beneficial bacteria the water has nothing to do with it. with a good amount of established live rock u might never see a spike in ammonia. I setup my seahorse tank in a day with sand and live rock from other tank and it's been like 3 weeks still no spike.
The sand can also be a motherload of detritus and absorbed nitrates. It could go either way. I decided to be safe rather than sorry and add new sand later.
Now that I think about it there was some gasses building up in the Sand I transferred so I rinsed it. But my source on the sand , Org.com - Only the best links ...
Seahorse.org has a lot of really strange and really bad advice on there. That isnt a place I would turn to for good info. I used to do a lot of reading on there and there are some really crazy people on there.