Only way of knowing how often you need to change water is by testing it, I would say 50% once a week, the bigger the water change, the more is removed. A must though is cleaning the gravel, as that is where all the poop and food gather which makes a lovely area for nitrATES to grow. When testing, Ammonia and nitrITES should be 0ppm,which means it has done the nitrogen cycle and nitrATES should be around 10ppm. I managed to keep 2 ranchu goldfish (and they are messy f**kers) in a 35 L tank, first 2 at 0ppm and nitrates at 5ppm. 2x20L water changes a week. Make sure you de-clorinate water before you put it in mind, otherwise its back to square 1 if it kills off the bacteria.
My gecko is ace, since getting it at 10 weeks old on the 9.9.10, it has gone from 2.4 g to 11.2g! Fat little git