Ok,there is a lot to be learnt about axies on here,I am a member and frequent user of this site http://www.caudata.org/forum/cgi-bin/show.cgi?793/793
Also Axolotl.org
I suggest you go there,everything you want to know is on there,and you can ask questions on the forums.
My first bit of advice is get rid of the gravel,they eat it and get blocked up,which is painful and kills.Also,a total water change is real bad,the cycling process causes ammonia,nitrite,nitrate,and ph to all go up and down,which can cause stress.Your tank looks a bit small,and no,a tunnel isnt neccessary but bare in mind axies dont like light and have no eyelids,so they do need some shelter/hiding places.Sorry to jump in and bag you,but I am apalled at how little people know about axies in this country,and the lack of want to know.Good on you for asking!!