That sounds a bit light on the feeding. 18 inches usually are in the 500-800 gm area typically most choose to feed 10% of the weight. With little guys I personally feed about 10% but every 5 days. I have a little guy currently about that size he is 675 gm and is getting a large weanling or small small (hate the rat size names) every 5-7 days (depending on if it is a bit small or a bit big.)

It sounds like a shed and sounds due pics would be helpful however. What is your temps and humidity like (warm, cool, and air)?