Odd question this, bear with me! Does anyone else's arm go red and itchy after spending some time dipped in tank water? I'm thinking I may have some sort of alergy to whatever is in the water if no one else get's it..
This came up either here on on APC recently. Basically it sounds like you have an allergic reaction and one thing that can cause it is duckweed. Apparently quite a few people are allergic to it. Make sure you wash your hands and arms thoroughly after every time you put your hands in may help a bit.
If you can't remove or find what's causing your reaction then you may have to start wearing gloves with long arms to prevent your skin touching the water.
I think you may find that's all there is! If you end up with the latex vet gloves (don't think of 'All Creatures Great and Small' while using them ) make sure you get ones that are unpowdered or you will add that to the tank.
Have you just tried making sure you wash any part of your skin that gets in touch with tank water very thoroughly as soon as you finish in the tank? If you used a Hypoallergenic handwash that might help too.
if you want some long gloves that can be thrown away, go to a farm shop like Mole Valley or Scats and get the long gloves (up to your shoulders) that are used for calfing. A box of them isn't that spenny and they aren't latex, which I prefer.
That could help