time frame...just do it whenever your ready. Just get rid of the gold fish first and clean it up, with a really good gravel vac, turn temp up to from the colder water and make sure parameters are good.
Depends what size it is. But before you go spend the cash, make sure it is not working properly. If it is not pushing out a lot of water on hi, try cleaning it out. That might help. If it dosent, then a new one may be in order.
