Trichozole and Flagyl are both trade names for metronidazole.
Vets tend to be quite concerned when dealing with areas they are not experienced in. I'm not saying the vets recommendations were right or wrong, just pointing out that it is an area of risk for them. They have a fairly hefty weight of responsibility when prescribing drugs and put themselves at risk when they are not too sure of the disease etc.
Ideally this is the time to brush up your knowledge or refer to another vet, but that's not always possible.
Metronidazole is listed as a potential carcinogen, so the vet was not being excessive recommending gloves are used. However it is a low risk, and like others have mentioned it is a fairly commonly prescribed human medication. It is prescribed where benefits outweigh the risk, the benefits of clearing up an infected root canal outweigh the slight possibility of increased risk of cancer. But risk should be avoided where possible - eg many countries have banned the use of metronidazole in food producing animals as it is an unnecessary risk.
Also there is a slight chance you may develop skin irritation from contact - unlikely but possible.
It's just about bottom (don't know if I can use the other word?) covering. Vets with fish knowledge are hard to find, so it pays to hang on to their number when you find one.