best advice would be
bigger tank
for that size tank, i'd say a firefish, damsel or chromis, you neeeeeeeed a fish that will not grow larger than 2 inches, and that is even pushing the limits of a 6 gal,
caution, 6 gal.s are very difficult, you need to stay on top of them at all times, test constantly and do maintenence as discovered, don't wait and say i'll get to it later, a day or even a half-day wiht any probs(no matter how small) could very well mean the diff. between success and fataliity
HTH AND GOOD LUCK