Leopard gecko smells

Question
Hi there, I currently have 2 6 month old leopard geckos.. These are my first leos, I keep them in a 4ft x 1ft heated aquarium with air holes in the lid.. I use sand substrate and artificial plants along with various wooden and clay hiding areas.
The problem is, they smell.. they are both feeding healthily every other day and seem in good health.. I have read that geckos shouldn't smell if kept clean which they have. I sieve out any poo daily.. I also have tried replacing the sand in their toilet area.. I'm a little lost what else to try? Any ideas if this could be due to the ventilation or anything?
Thanks for reading, any advice would be much appreciated,
Glen

Answer
Hello Glen, I advise removing the sand it can be fatal if ingested, which your leo will try to eat it if the leo is lacking calcium. The sand will get wet in their insides then gum up in their intestines which is almost fatal everytime. So remove that because it also holds in smells too believe it or not. I use a type of carpet for my Leos and it is actually made to stick to the back of a hermit crab tank for them to climb on but it works just as well as leo substrate, they can't eat it and it looks natural. Here is a link to the product at petco (I am not sure if it is sold any where else)
Oh and best of all it barely cost a thing so its easy to replace if it gets too dirty. http://www.petco.com/product/12007/T-Rex-Crab-Island-Climbing-Background.aspx

As for whats causing the smell I am not sure. Do you bathe your leos often? They should have a bath about once a month because they can smell themselves too. Leopard Geckos don't usually like water so they might get mad at you at first but fill a small rubbermaid tub with enough lukewarm water to cover his knees and nothing else. They aren't swimmers so keep an eye on him EVERY second he is in their. Just like bathing a baby human don't leave unattended.
It could also be ventilation so maybe try a metal screen over the top. Good luck and let me know how it goes. If none of these work or you have more questions let me know.