Easier to do when it's empty. Especially with a larger/deeper tank. But you could. Just easier to get set up and move/change things around without the water to deal with. Also could always drain it some which won't mean that you're starting over again completely from scratch. But it would be a bunch of water to be moving. Not sure how you're set up as far as that goes.
Will depend too on what you want to do. If more simple that you can get to and arrange relatively easily, then not too bad. If you're trying to do something real intricate, then harder. Either way, especially with a first tank, you're probably going to end up moving things around some and you'll need to maintain things anyway when it's full so kinda get used to getting your sleeves wet.
No real big deal either way.
I'm not a big fan of the Eco-Complete myself. I'd save the money and just go with sand of one sort or another.
Last edited by Mike A.; 05-21-2019 at 06:31 AM.