We definitely needed a dehumidifier. I think it cost about $200 at Sears. It is a 35 pint one, and it has to be emptied twice a day. The only time it doesn't have to be run is if the window unit A/C is running, and that does pretty much the same thing. If it's left off, or auto-shuts off due to a full bucket, the humidity builds up really quickly. I would definitely add this to the budget of any indoor pond. You can get the ones that don't have to be emptied; they have a little pump and a tube you stick out the window, but I read too many reports of the pumps failing, and then you've spent an extra couple hundred bucks for nothing.