I have just put a deposit on a 180litre tank for my comet and shubunkin, I haven't got it yet but wondered of anyone had any advice for decorating it. What is better sand or gravel? And should I theme it? Also should I cycle it before I decorate or should I decorate it first? Any suggestion will be appreciated. Thanks.

Sand should be avoided when keeping goldfish since they will dig in the substrate and may end up swallowing some sand.

I always decorate my tanks before I cycle them, so that they're "complete" and ready for the fish to be introduced, but it doesn't matter in the end

When it comes to theme, I would decide only largely how I want it to look, for example, should it be a natural tank with only rocks, roots and plants or do I want it to have some plastic ornaments or... you know. But it's a good idea to avoid colored gravel, it's a bit of an eyesore both for people and fish

You should avoid ornaments that goldfish can swim into, or that have sharp edges. Generally a nice forest of plants will do just fine with that. The larger fish rarely want to hide anyway