With such a small tank we can get away without a skimmer but you have to keep up your water changes. I had a 29g regular tank then upgraded to a 55g sw
reef. the 29g when i had it didn't have a skimmer until later. I never had any water issues.
I ended up buying a remora skimmer that i still use now on the 55g. Not many people on here like the remora but I've never had any problem with it. I think they make a baby size remora too.
heres a good link.
BioCube 29 Package Deal - Aquarium Filtration Upgrades & Supplies
They have alot of the tweaking stuff for cubes.