Correct me if I'm wrong but aren't GloFish Zebra Danios? If so, Personally I think a 6 gallon tank is too small for them, they are way to active for anythig smaller than 10-15 gallons. You are going to have to find another tank for your fish or retun them to your LFS they ARE NOT compatible with your newt. The only fish, if any, that you can try are white cloud minnows, but your tank is a bit on the small side. And unless you have dwarf crayfish, which your newt may eat or injure your newt whichever comes first, then a 6 Gallon is DEFINATLY too small for any any crayfish. Plus you risk them eating or injuring your newt. Ghost shrimp are a better route to take, they are easy to find and much cheaper to replace if your newt snacks on them.
Fire Belly Newts are usually semi-aquatic if your newt enjoys water more than land I wouldn't worry too much. My newts rarely go in the water to swim, they usually hang out by the waterfall/river of my tank.
What kind of lighting do you have? If you are using one of the prefab eclipse tanks by marineland with an incadecent bulb you are asking for tempature issues. Use either a CF bulb or flind some sort of small flourecent bulb.
Ideally you want to get the test kit ASAP, in the mean time do frequent water changes daily to bi-daily, if possible. I'm not conviced that the driftwood is the only problem, but I'm not denying it may of had something to do with your loss. If this is a newly setup tank (which I'm assuming it is) then you may have cycling issues to resolve aswell. Either way doing water changes will help.
Live plants are a good idea, you may want to look into elodea as well newts seem to enjoy it, and a boarder leafed species like Anubia sp. Make sure your live plants (and maybe even your driftwood) weren't pre-treated with anything, i.e. copper, some fish are sensitive to copper, and that would be a VERY good reason why your crayfish died. I dont know what kind of effect it would have on your newt but personally I wouldnt chance it.
I hope this helps, please feel free to ask more questions. Also the people on Caudata.org forums are VERY knowledgable with questions about FBN and other species of newts and salamanders, it wouldnt hurt to get their opinion on what is best for your newt. Its where I go for newt related info and problems, I come here for everything else.
Joe
just doesn't work. The crayfish are bound to hurt the poor lil guy or girl. Also, the fish you have in there don't mix either, they need higher temperatures while Fire Bellied Newts, Chinese or Japanese need cooler water. Fire Bellied Newts are still very social creatures, and will suffer from loneliness. So I suggest buying a bigger tank, more newts and just putting all newts in the bigger tank. It may be expensive and a bit of a hassle, but it's best for your fish AND your newt.