it helps lower stress help gill function promotes disease recovery but do put to much in it
http/www.aquariumfish.net/information/aquarium_salt.htm
Helps gill function? I would like to see some evidence for that.
The old belief was that because FW fish are always having to maintain a higher dissolved salt level in their body than in the water around them that increasing the dissolved salt level would aid them in their osmoregulation. However, the fish have evolved over millions of years to have their osmoregulatory systems cope with their natural (low salt) water conditions, and some fish handle salt badly (as noted in the link).
Salt was mainly used back when "old water" was believed to be very good for the aquarium (much like carbon). The salt would reduce nitrite (and I suppose, to some extenet, nitrate) toxicity for fish whereas we now do this far better with water changes. Aquarium salt is not in any way necessary as a continuous addition to the water of a FW tank.
Back to the original topic, the risks are most likely very small. I can think of very few (if any) cases of someone on this forum (which gives a large n number) having any health problems associated with the tank water. One hears of fish biting, electrocuting (CFC...) and some marine fish and inverts injecting venom but very rarely, if ever, any problems from the water.
I myself take no precautions with my tanks, washing my hands neither before, or after, working on them. The only problem I have ever noticed is if my Eczema has broken the skin on my hands and I work on the marine tanks too long. I presume that the salt water draws water from the area worsening the condition.