When I set my 29 up, I had major GW issues. Turns out direct sunlight caused it every time, put a background on it, it never comes back untill I leave the background off for a few days. I don't know why but without fail, it always happened.
And no, you cant use a blacklight, it doesn't emit the same UV radiation that can actually kill living tissue. UV units aren't cheap usually, petsmart does have a cheap solution for like 30-50 bucks depending on size, can do wonders too.