im upgrading to a 30g tank from a 20g can i use the water from the 20g and just put it in my 30g then do water changes / topups. just wondering cos dont really want to do another 2 -3 week cycle but if it is best to i will.
No.
Not unless you also tranfer the filter media, gravel and ornamets ... where all the nitrifying bacteria live ... they don't live in the water.
Short answer. No.
If you're transferring everything, then you shouldn't have a cylce. It might be a good idea to have some Amquel on hand in case you get a small ammonia spike, though.
