Fu manchus have the habit of getting bacterial infections, which is usually a direct result of a dirty tank. As far an anglers go, they are above average in terms of "hardiness", but i'm not 100% sure since i've never owned one.
As far as 30 ppm nitrates killing your fish, or nitrates in general, it depends on the speed with which your nitrates rise. I had a tank for three years that due to several factors, especially certain lifestock that would always disrupt my sand bed, always had the highest nitrates I had ever seen. My nitrates in this tank were off the chart, you know, the dark burgundy at the top of the scale, that was my tank. We're talking as bad as it can get, and I never once lost a fish b/c the nitrates never "spiked", they rose slowly over months and then stayed constant at that level. Even trying 10% water changes every day for 10 days did nothing to lessen the nitrates (i think it had to do w/ my water source as well). When I purchased new fish, they were slowly acclimated over the course of several hours and they too, never died, in three years of owning the tank.
So I really wouldn't stress over nitrates too much at all unless theyre going from 0 to 90 in a day or two, and back, and repeating. The spikes are the fish killers, not the high levels themselves.
